Implement rehosting

There is a game command called `Rehost` which is sent by the game when a player clicks the rehost button. This is currently ignored, and as far as I can tell, simply closes the game for the user after sending the message. However, we could have the server respond by automatically triggering a `game_host` command, and maybe even go so far as to trigger `game_join` commands for every player that was connected to the lobby. I think we don't need to worry too much about game settings as these will be remembered on the client side, and sent by the host again when the game re-opens. We might need to remember the mapname though.