MM-30048 added thread related socket messages (#16234)

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

store/sqlstore/post_store.go

@@ -1976,7 +1976,7 @@ func (s *SqlPostStore) updateThreadsFromPosts(transaction *gorp.Transaction, pos
 		if thread, found := threadByRoot[rootId]; !found {
 			// calculate participants
 			var participants model.StringArray
-			if _, err := transaction.Select(&participants, "SELECT DISTINCT UserId FROM Posts WHERE RootId=:RootId", map[string]interface{}{"RootId": rootId}); err != nil {
+			if _, err := transaction.Select(&participants, "SELECT DISTINCT UserId FROM Posts WHERE RootId=:RootId OR Id=:RootId", map[string]interface{}{"RootId": rootId}); err != nil {
 				return err
 			}
 			// calculate reply count