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.

The issue names no files, tests, or entry points; start by locating the room-deletion path and the server/client code that provides room lists. Done means clients and the server can learn when rooms are deleted without relying on the proposed periodic polling alternatives.

Written by the indexing model from the issue text.