Make room id shorter

[erikbonner]

Great game!

We often play this on two devices between which it's not so easy to send the lobby id - so we manually enter the id on the second device. This is a bit of work when the lobby id is 12 characters long.

An id length of 12 is a bit overkill. Each character in the id can be one of [A-Za-z0-9], which gives you a total of 62^12 possible lobby ids. Although this game will hopefully have plenty of parallel lobbies in future, this may still be a bit much :)

Could the lobby id length be reduced to 4 or 5? This should be sufficient to avoid collisions and would be much easier to type manually.