Accept votes to advance game at /end-answers and /end-judging endpoints

The previous logic was far too strict, where the game would advance
only after every registered player had submitted. That made it
possible for the game to become totally stuck with just a single
player that left the game, (or even that accidentally reloaded the
game in a way that lost their previous session).

Now there are endpoints for players to vote that they would like to
advance past either the "answers" or "judging" phases of the game, and
the game will advance as soon as a majority of the registered players
have voted.

This does still make it possible for the game to get stuck if a
sufficient number of players stop playing, but it's at least less
likely to happen. And we've got plans to improve this situation
further soon as well.