Address room ID collision issue

[nikolas]

Currently, the "Make Room" procedure can overwrite existing rooms - the room ID is generated at random. Let's make sure the room is empty before using that ID for a new room.

Which begs the question - when and how are rooms made empty? The policy itself is very flexible - e.g. after a day or so of inactivity we can clean up room data. But how exactly will this happen on the technical side: a cron job? A celery task? Obviously, let's keep things as simple as possible.

[nikolas]

Proposal by Evan: we have a user count in the room, we can catch the event where a room's population reaches 0, and consider that room to be "empty" and ready for cleanup.

I'll implement this and see how it feels.

[nikolas]

Although emptying the room when everyone leaves may be slightly simpler on the technical side, it sounds like what we want is a timed time-out. So, I'll set something up to clear the room after 24 hours of inactivity, to start.