Add a button to Stop an in-progress game.

Allow the game host to Stop a game via a button that appears where the Start Game button did.  (I did consider making it a chat command, but that's probably **too** hidden for most users.)

The intent is to allow the host to stop a recently-started game, in case they realise that the wrong options had been chosen or if they want to wait for another player to join, or whatever.

At present it will allow the host to reset the game at any time.  If it gets abused, it could be restricted to only allow it during the first few rounds.

(An alternative to this might be to allow the host to change options during play, although that could get complicated to allow switching decks [if eg. they forgot to include one].)