Restrict session modification to the game server

[KMayne]

Users will be able to create a new session of a game but only the game server should be able to modify game sessions.
Perhaps don't allow deletion of sessions?

[ashleydavies]

Deletion is a bit dodgy anyway. I'd prefer just NULLing the state and maybe even allowing them to allocate a winner - minimal change and we could get lots of statistics out of it.