fix: #8127 user join system message duplicated

- Now showing user-join message for chat owner in addition to the newly
  added user
- If a modal already exists for a room when createModal is called, don't
  return null, return the modal

src/messaging/rooms.js

@@ -53,6 +53,7 @@ module.exports = function (Messaging) {
 			db.sortedSetAdd('chat:room:' + roomId + ':uids', now, uid),
 		]);
 		await Promise.all([
+			Messaging.addSystemMessage('user-join', uid, roomId),	// chat owner should also get the user-join system message
 			Messaging.addUsersToRoom(uid, toUids, roomId),
 			Messaging.addRoomToUsers(roomId, [uid].concat(toUids), now),
 		]);