MM-30338 Add feature flag for collapsedthreads (#16598)

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/notification.go

@@ -420,7 +420,7 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 
 	a.Publish(message)
 	// If this is a reply in a thread, notify participants
-	if *a.Config().ServiceSettings.CollapsedThreads != model.COLLAPSED_THREADS_DISABLED && post.RootId != "" {
+	if a.Config().FeatureFlags.CollapsedThreads && *a.Config().ServiceSettings.CollapsedThreads != model.COLLAPSED_THREADS_DISABLED && post.RootId != "" {
 		thread, err := a.Srv().Store.Thread().Get(post.RootId)
 		if err != nil {
 			return nil, errors.Wrapf(err, "cannot get thread %q", post.RootId)