Detect when rooms get deleted

Right now, rooms get deleted through mongodb. This separation prevents the server or client from knowing if a room got deleted.
What we could do is let the client just update rooms every 5 minutes, not a fun solution.
We could also have the server watch every minute if a room got deleted. This would require it store in memory the list of room IDs and periodically check if a room got deleted.
We could instead just have the server send everyone the updated list of rooms every 2-5 minutes.