Add a closing message and make the reboot button functional

Add a closing message and make the reboot button functional

Also fix a bunch of state transitions, (joining the game in-progress,
rebooting from anywhere, reloading from anywhere).

With the sloppy DOM-manipulation approach I'm using it's pretty
painful to get all of those right, (could have used React or
something, but that has its own set of problems), so I won't promise
that everything is perfect, but I think it's pretty close.