[PLT-7362] Option to add user to channel if mentioned user is not currently in the channel (#7619)

* Option to add user to channel if mentioned user is not currently in the channel

* instead of link from server, just add component on client side to add channel member

* change implementation using post.props

* do clean up and add test

* sanitize post.props['add_channel_member'] on post creation

* move sanitize to app.CreatePost and also apply to app.UpdatePost

2 files changed, 33 insertions, 4 deletions

diff --git a/app/notification.go b/app/notification.go
index e11218faa..bbd305b05 100644
--- a/app/notification.go
+++ b/app/notification.go
@@ -94,7 +94,7 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 				outOfChannelMentions := result.Data.([]*model.User)
 				if channel.Type != model.CHANNEL_GROUP {
 					a.Go(func() {
-						a.sendOutOfChannelMentions(sender, post, team.Id, outOfChannelMentions)
+						a.sendOutOfChannelMentions(sender, post, channel.Type, outOfChannelMentions)
 					})
 				}
 			}
@@ -723,7 +723,7 @@ func (a *App) getMobileAppSessions(userId string) ([]*model.Session, *model.AppE
 	}
 }
 
-func (a *App) sendOutOfChannelMentions(sender *model.User, post *model.Post, teamId string, users []*model.User) *model.AppError {
+func (a *App) sendOutOfChannelMentions(sender *model.User, post *model.Post, channelType string, users []*model.User) *model.AppError {
 	if len(users) == 0 {
 		return nil
 	}
@@ -734,26 +734,51 @@ func (a *App) sendOutOfChannelMentions(sender *model.User, post *model.Post, tea
 	}
 	sort.Strings(usernames)
 
+	var userIds []string
+	for _, user := range users {
+		userIds = append(userIds, user.Id)
+	}
+
 	T := utils.GetUserTranslations(sender.Locale)
 
+	var localePhrase string
+	if channelType == model.CHANNEL_OPEN {
+		localePhrase = T("api.post.check_for_out_of_channel_mentions.link.public")
+	} else if channelType == model.CHANNEL_PRIVATE {
+		localePhrase = T("api.post.check_for_out_of_channel_mentions.link.private")
+	}
+
+	ephemeralPostId := model.NewId()
 	var message string
-	if len(usernames) == 1 {
+	if len(users) == 1 {
 		message = T("api.post.check_for_out_of_channel_mentions.message.one", map[string]interface{}{
 			"Username": usernames[0],
+			"Phrase":   localePhrase,
 		})
 	} else {
 		message = T("api.post.check_for_out_of_channel_mentions.message.multiple", map[string]interface{}{
-			"Usernames":    strings.Join(usernames[:len(usernames)-1], ", "),
+			"Usernames":    strings.Join(usernames[:len(usernames)-1], ", @"),
 			"LastUsername": usernames[len(usernames)-1],
+			"Phrase":       localePhrase,
 		})
 	}
 
+	props := model.StringInterface{
+		model.PROPS_ADD_CHANNEL_MEMBER: model.StringInterface{
+			"post_id":   ephemeralPostId,
+			"usernames": usernames,
+			"user_ids":  userIds,
+		},
+	}
+
 	a.SendEphemeralPost(
 		post.UserId,
 		&model.Post{
+			Id:        ephemeralPostId,
 			ChannelId: post.ChannelId,
 			Message:   message,
 			CreateAt:  post.CreateAt + 1,
+			Props:     props,
 		},
 	)
 
diff --git a/app/post.go b/app/post.go
index f12866217..4d91e8122 100644
--- a/app/post.go
+++ b/app/post.go
@@ -104,6 +104,8 @@ func (a *App) CreatePostMissingChannel(post *model.Post, triggerWebhooks bool) (
 }
 
 func (a *App) CreatePost(post *model.Post, channel *model.Channel, triggerWebhooks bool) (*model.Post, *model.AppError) {
+	post.SanitizeProps()
+
 	var pchan store.StoreChannel
 	if len(post.RootId) > 0 {
 		pchan = a.Srv.Store.Post().Get(post.RootId)
@@ -273,6 +275,8 @@ func (a *App) SendEphemeralPost(userId string, post *model.Post) *model.Post {
 }
 
 func (a *App) UpdatePost(post *model.Post, safeUpdate bool) (*model.Post, *model.AppError) {
+	post.SanitizeProps()
+
 	var oldPost *model.Post
 	if result := <-a.Srv.Store.Post().Get(post.Id); result.Err != nil {
 		return nil, result.Err