MM-37372 Do not autofollow threads started by webhooks/bots for user who created them (#18276)

* Do not autofollow threads started by webhooks/bots for user who created them

* Add test

app/notification.go

@@ -201,9 +201,11 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 	if *a.Config().ServiceSettings.ThreadAutoFollow && post.RootId != "" {
 		var rootMentions *ExplicitMentions
 		if parentPostList != nil {
-			threadParticipants[parentPostList.Posts[parentPostList.Order[0]].UserId] = true
+			rootPost := parentPostList.Posts[parentPostList.Order[0]]
+			if rootPost.GetProp("from_webhook") != "true" {
+				threadParticipants[rootPost.UserId] = true
+			}
 			if channel.Type != model.ChannelTypeDirect {
-				rootPost := parentPostList.Posts[parentPostList.Order[0]]
 				rootMentions = getExplicitMentions(rootPost, keywords, groups)
 				for id := range rootMentions.Mentions {
 					threadParticipants[id] = true