Add "back to room" functionality

[Nick-Gabe]

Sometimes when you exit a room, you just wanted to change your username.
Changing the username on the lobby is a bit hard since it also maps to the superviz user, so it requires the user to exit and come back again. However at the moment we don't have a "back again" feature.

One example of implementation is that when you exit a room, it should still maintain a query ?room=ID, so the "join" button appears and the user can go back if they want. Also this makes necessary for the "new game" button to always be visible on the home page.