MM-9072/MM-10185 Force-convert the encoding of OpenGraph metadata to UTF-8 (#8631)

* Force-convert non-UTF8 HTML to UTF8 before opengraph processing

* Split the force-encoding function

* Add benchmark Test for the forceHTMLEncodingToUTF8()

```
Running tool: /home/comet/go-v1.9.2/bin/go test -benchmem -run=^$ github.com/mattermost/mattermost-server/app -bench ^BenchmarkForceHTMLEncodingToUTF8$

[03:32:58 KST 2018/04/21] [INFO] (github.com/mattermost/mattermost-server/app.TestMain:28) -test.run used, not creating temporary containers
goos: linux
goarch: amd64
pkg: github.com/mattermost/mattermost-server/app
BenchmarkForceHTMLEncodingToUTF8/with_converting-4         	  100000	     11201 ns/op	   18704 B/op	      32 allocs/op
BenchmarkForceHTMLEncodingToUTF8/without_converting-4      	  300000	      3931 ns/op	    4632 B/op	      13 allocs/op
PASS
ok  	github.com/mattermost/mattermost-server/app	2.703s
Success: Benchmarks passed.
```

* Remove an unnecessary constraint

* Add pre-check if content-type header is already utf-8

* Move the checking for utf-8 into forceHTMLEncodingToUTF8() for testing

* Revert df3f347213faa0d023c26d201fa6531f46391086..HEAD, without Gopkg.lock

1 files changed, 352 insertions, 0 deletions

diff --git a/vendor/golang.org/x/text/encoding/htmlindex/tables.go b/vendor/golang.org/x/text/encoding/htmlindex/tables.go
new file mode 100644
index 000000000..9d6b4315c
--- /dev/null
+++ b/vendor/golang.org/x/text/encoding/htmlindex/tables.go
@@ -0,0 +1,352 @@
+// Code generated by running "go generate" in golang.org/x/text. DO NOT EDIT.
+
+package htmlindex
+
+type htmlEncoding byte
+
+const (
+	utf8 htmlEncoding = iota
+	ibm866
+	iso8859_2
+	iso8859_3
+	iso8859_4
+	iso8859_5
+	iso8859_6
+	iso8859_7
+	iso8859_8
+	iso8859_8I
+	iso8859_10
+	iso8859_13
+	iso8859_14
+	iso8859_15
+	iso8859_16
+	koi8r
+	koi8u
+	macintosh
+	windows874
+	windows1250
+	windows1251
+	windows1252
+	windows1253
+	windows1254
+	windows1255
+	windows1256
+	windows1257
+	windows1258
+	macintoshCyrillic
+	gbk
+	gb18030
+	big5
+	eucjp
+	iso2022jp
+	shiftJIS
+	euckr
+	replacement
+	utf16be
+	utf16le
+	xUserDefined
+	numEncodings
+)
+
+var canonical = [numEncodings]string{
+	"utf-8",
+	"ibm866",
+	"iso-8859-2",
+	"iso-8859-3",
+	"iso-8859-4",
+	"iso-8859-5",
+	"iso-8859-6",
+	"iso-8859-7",
+	"iso-8859-8",
+	"iso-8859-8-i",
+	"iso-8859-10",
+	"iso-8859-13",
+	"iso-8859-14",
+	"iso-8859-15",
+	"iso-8859-16",
+	"koi8-r",
+	"koi8-u",
+	"macintosh",
+	"windows-874",
+	"windows-1250",
+	"windows-1251",
+	"windows-1252",
+	"windows-1253",
+	"windows-1254",
+	"windows-1255",
+	"windows-1256",
+	"windows-1257",
+	"windows-1258",
+	"x-mac-cyrillic",
+	"gbk",
+	"gb18030",
+	"big5",
+	"euc-jp",
+	"iso-2022-jp",
+	"shift_jis",
+	"euc-kr",
+	"replacement",
+	"utf-16be",
+	"utf-16le",
+	"x-user-defined",
+}
+
+var nameMap = map[string]htmlEncoding{
+	"unicode-1-1-utf-8":   utf8,
+	"utf-8":               utf8,
+	"utf8":                utf8,
+	"866":                 ibm866,
+	"cp866":               ibm866,
+	"csibm866":            ibm866,
+	"ibm866":              ibm866,
+	"csisolatin2":         iso8859_2,
+	"iso-8859-2":          iso8859_2,
+	"iso-ir-101":          iso8859_2,
+	"iso8859-2":           iso8859_2,
+	"iso88592":            iso8859_2,
+	"iso_8859-2":          iso8859_2,
+	"iso_8859-2:1987":     iso8859_2,
+	"l2":                  iso8859_2,
+	"latin2":              iso8859_2,
+	"csisolatin3":         iso8859_3,
+	"iso-8859-3":          iso8859_3,
+	"iso-ir-109":          iso8859_3,
+	"iso8859-3":           iso8859_3,
+	"iso88593":            iso8859_3,
+	"iso_8859-3":          iso8859_3,
+	"iso_8859-3:1988":     iso8859_3,
+	"l3":                  iso8859_3,
+	"latin3":              iso8859_3,
+	"csisolatin4":         iso8859_4,
+	"iso-8859-4":          iso8859_4,
+	"iso-ir-110":          iso8859_4,
+	"iso8859-4":           iso8859_4,
+	"iso88594":            iso8859_4,
+	"iso_8859-4":          iso8859_4,
+	"iso_8859-4:1988":     iso8859_4,
+	"l4":                  iso8859_4,
+	"latin4":              iso8859_4,
+	"csisolatincyrillic":  iso8859_5,
+	"cyrillic":            iso8859_5,
+	"iso-8859-5":          iso8859_5,
+	"iso-ir-144":          iso8859_5,
+	"iso8859-5":           iso8859_5,
+	"iso88595":            iso8859_5,
+	"iso_8859-5":          iso8859_5,
+	"iso_8859-5:1988":     iso8859_5,
+	"arabic":              iso8859_6,
+	"asmo-708":            iso8859_6,
+	"csiso88596e":         iso8859_6,
+	"csiso88596i":         iso8859_6,
+	"csisolatinarabic":    iso8859_6,
+	"ecma-114":            iso8859_6,
+	"iso-8859-6":          iso8859_6,
+	"iso-8859-6-e":        iso8859_6,
+	"iso-8859-6-i":        iso8859_6,
+	"iso-ir-127":          iso8859_6,
+	"iso8859-6":           iso8859_6,
+	"iso88596":            iso8859_6,
+	"iso_8859-6":          iso8859_6,
+	"iso_8859-6:1987":     iso8859_6,
+	"csisolatingreek":     iso8859_7,
+	"ecma-118":            iso8859_7,
+	"elot_928":            iso8859_7,
+	"greek":               iso8859_7,
+	"greek8":              iso8859_7,
+	"iso-8859-7":          iso8859_7,
+	"iso-ir-126":          iso8859_7,
+	"iso8859-7":           iso8859_7,
+	"iso88597":            iso8859_7,
+	"iso_8859-7":          iso8859_7,
+	"iso_8859-7:1987":     iso8859_7,
+	"sun_eu_greek":        iso8859_7,
+	"csiso88598e":         iso8859_8,
+	"csisolatinhebrew":    iso8859_8,
+	"hebrew":              iso8859_8,
+	"iso-8859-8":          iso8859_8,
+	"iso-8859-8-e":        iso8859_8,
+	"iso-ir-138":          iso8859_8,
+	"iso8859-8":           iso8859_8,
+	"iso88598":            iso8859_8,
+	"iso_8859-8":          iso8859_8,
+	"iso_8859-8:1988":     iso8859_8,
+	"visual":              iso8859_8,
+	"csiso88598i":         iso8859_8I,
+	"iso-8859-8-i":        iso8859_8I,
+	"logical":             iso8859_8I,
+	"csisolatin6":         iso8859_10,
+	"iso-8859-10":         iso8859_10,
+	"iso-ir-157":          iso8859_10,
+	"iso8859-10":          iso8859_10,
+	"iso885910":           iso8859_10,
+	"l6":                  iso8859_10,
+	"latin6":              iso8859_10,
+	"iso-8859-13":         iso8859_13,
+	"iso8859-13":          iso8859_13,
+	"iso885913":           iso8859_13,
+	"iso-8859-14":         iso8859_14,
+	"iso8859-14":          iso8859_14,
+	"iso885914":           iso8859_14,
+	"csisolatin9":         iso8859_15,
+	"iso-8859-15":         iso8859_15,
+	"iso8859-15":          iso8859_15,
+	"iso885915":           iso8859_15,
+	"iso_8859-15":         iso8859_15,
+	"l9":                  iso8859_15,
+	"iso-8859-16":         iso8859_16,
+	"cskoi8r":             koi8r,
+	"koi":                 koi8r,
+	"koi8":                koi8r,
+	"koi8-r":              koi8r,
+	"koi8_r":              koi8r,
+	"koi8-ru":             koi8u,
+	"koi8-u":              koi8u,
+	"csmacintosh":         macintosh,
+	"mac":                 macintosh,
+	"macintosh":           macintosh,
+	"x-mac-roman":         macintosh,
+	"dos-874":             windows874,
+	"iso-8859-11":         windows874,
+	"iso8859-11":          windows874,
+	"iso885911":           windows874,
+	"tis-620":             windows874,
+	"windows-874":         windows874,
+	"cp1250":              windows1250,
+	"windows-1250":        windows1250,
+	"x-cp1250":            windows1250,
+	"cp1251":              windows1251,
+	"windows-1251":        windows1251,
+	"x-cp1251":            windows1251,
+	"ansi_x3.4-1968":      windows1252,
+	"ascii":               windows1252,
+	"cp1252":              windows1252,
+	"cp819":               windows1252,
+	"csisolatin1":         windows1252,
+	"ibm819":              windows1252,
+	"iso-8859-1":          windows1252,
+	"iso-ir-100":          windows1252,
+	"iso8859-1":           windows1252,
+	"iso88591":            windows1252,
+	"iso_8859-1":          windows1252,
+	"iso_8859-1:1987":     windows1252,
+	"l1":                  windows1252,
+	"latin1":              windows1252,
+	"us-ascii":            windows1252,
+	"windows-1252":        windows1252,
+	"x-cp1252":            windows1252,
+	"cp1253":              windows1253,
+	"windows-1253":        windows1253,
+	"x-cp1253":            windows1253,
+	"cp1254":              windows1254,
+	"csisolatin5":         windows1254,
+	"iso-8859-9":          windows1254,
+	"iso-ir-148":          windows1254,
+	"iso8859-9":           windows1254,
+	"iso88599":            windows1254,
+	"iso_8859-9":          windows1254,
+	"iso_8859-9:1989":     windows1254,
+	"l5":                  windows1254,
+	"latin5":              windows1254,
+	"windows-1254":        windows1254,
+	"x-cp1254":            windows1254,
+	"cp1255":              windows1255,
+	"windows-1255":        windows1255,
+	"x-cp1255":            windows1255,
+	"cp1256":              windows1256,
+	"windows-1256":        windows1256,
+	"x-cp1256":            windows1256,
+	"cp1257":              windows1257,
+	"windows-1257":        windows1257,
+	"x-cp1257":            windows1257,
+	"cp1258":              windows1258,
+	"windows-1258":        windows1258,
+	"x-cp1258":            windows1258,
+	"x-mac-cyrillic":      macintoshCyrillic,
+	"x-mac-ukrainian":     macintoshCyrillic,
+	"chinese":             gbk,
+	"csgb2312":            gbk,
+	"csiso58gb231280":     gbk,
+	"gb2312":              gbk,
+	"gb_2312":             gbk,
+	"gb_2312-80":          gbk,
+	"gbk":                 gbk,
+	"iso-ir-58":           gbk,
+	"x-gbk":               gbk,
+	"gb18030":             gb18030,
+	"big5":                big5,
+	"big5-hkscs":          big5,
+	"cn-big5":             big5,
+	"csbig5":              big5,
+	"x-x-big5":            big5,
+	"cseucpkdfmtjapanese": eucjp,
+	"euc-jp":              eucjp,
+	"x-euc-jp":            eucjp,
+	"csiso2022jp":         iso2022jp,
+	"iso-2022-jp":         iso2022jp,
+	"csshiftjis":          shiftJIS,
+	"ms932":               shiftJIS,
+	"ms_kanji":            shiftJIS,
+	"shift-jis":           shiftJIS,
+	"shift_jis":           shiftJIS,
+	"sjis":                shiftJIS,
+	"windows-31j":         shiftJIS,
+	"x-sjis":              shiftJIS,
+	"cseuckr":             euckr,
+	"csksc56011987":       euckr,
+	"euc-kr":              euckr,
+	"iso-ir-149":          euckr,
+	"korean":              euckr,
+	"ks_c_5601-1987":      euckr,
+	"ks_c_5601-1989":      euckr,
+	"ksc5601":             euckr,
+	"ksc_5601":            euckr,
+	"windows-949":         euckr,
+	"csiso2022kr":         replacement,
+	"hz-gb-2312":          replacement,
+	"iso-2022-cn":         replacement,
+	"iso-2022-cn-ext":     replacement,
+	"iso-2022-kr":         replacement,
+	"utf-16be":            utf16be,
+	"utf-16":              utf16le,
+	"utf-16le":            utf16le,
+	"x-user-defined":      xUserDefined,
+}
+
+var localeMap = []htmlEncoding{
+	windows1252, // und_Latn
+	windows1256, // ar
+	windows1251, // ba
+	windows1251, // be
+	windows1251, // bg
+	windows1250, // cs
+	iso8859_7,   // el
+	windows1257, // et
+	windows1256, // fa
+	windows1255, // he
+	windows1250, // hr
+	iso8859_2,   // hu
+	shiftJIS,    // ja
+	windows1251, // kk
+	euckr,       // ko
+	windows1254, // ku
+	windows1251, // ky
+	windows1257, // lt
+	windows1257, // lv
+	windows1251, // mk
+	iso8859_2,   // pl
+	windows1251, // ru
+	windows1251, // sah
+	windows1250, // sk
+	iso8859_2,   // sl
+	windows1251, // sr
+	windows1251, // tg
+	windows874,  // th
+	windows1254, // tr
+	windows1251, // tt
+	windows1251, // uk
+	windows1258, // vi
+	gb18030,     // zh-hans
+	big5,        // zh-hant
+}
+
+const locales = "und_Latn ar ba be bg cs el et fa he hr hu ja kk ko ku ky lt lv mk pl ru sah sk sl sr tg th tr tt uk vi zh-hans zh-hant"