[MM-21919] Add use_channel_mentions permission (#13781)

* MM-21919 Add channel_mention permission

* MM-21919 Fix emojis migration test

* Enforce Channel Mentions on the in the posts api

* MM-21919 Rename permission to use_channel_mentions

Allow posts with channel mentions to still be posted without the permission just don't send notifications to users

* MM-21919 Add tests for a post with @all and @here

* MM-21919 Add use channel mentions to all roles that have create post

* MM-21919 Update app_test to include use_channel_mentions permission in default permissions

app/notification.go

@@ -624,6 +624,10 @@ func getMentionsEnabledFields(post *model.Post) model.StringArray {
 
 // allowChannelMentions returns whether or not the channel mentions are allowed for the given post.
 func (a *App) allowChannelMentions(post *model.Post, numProfiles int) bool {
+	if !a.HasPermissionToChannel(post.UserId, post.ChannelId, model.PERMISSION_USE_CHANNEL_MENTIONS) {
+		return false
+	}
+
 	if post.Type == model.POST_HEADER_CHANGE || post.Type == model.POST_PURPOSE_CHANGE {
 		return false
 	}