Change game start UI to allow owner to confirm players

[eltoder]

The goal is to give owners some control over who joins their game. This solves the bot issue, but also issues like someone went AWOL after joining.

Conversely, it would be nice for non-owners to be able to join the game's lobby and see what's going on before committing to join the game.

[eltoder]

I'm thinking of adding checkboxes next to player names. When a new player comes in, it starts as unchecked. The host can check all with one click, or check them individually. When the game is started with "play again", all previous players are automatically checked.