Add a button to advance from prompt voting to playing the actual game

Once a majority of the players have voted a new button appears (for
everyone) that when pressed tells the server that it's time to start
the game.

Then, when the client receives a "start" event from the server with
the prompt the server chose, the client replaces the entire interface
with a form for entering the responses to the prompt.

The submit button for that form is currently inert.