Improve handling for players leaving/joining multiplayer games during votes

[GooberRF]

This PR adjusts voting in the following ways:

• When all players have left the server, an active vote is now automatically canceled.
• When the person who called a vote leaves the server, the vote is now automatically canceled.
• Newly joined players can now participate in the vote.
• Players who leave during a vote are now automatically removed from influence on the vote outcome - if they didn't vote, they're no longer considered; if they did vote, that vote is removed/ignored.

Previously, the behaviour for players who join/leave during a vote was relatively unintuitive - ie. players who joined during a vote were excluded from participating (despite receiving a reminder to do so), and a vote was still being expected from players who had already left the game.

This new approach is much more intuitive and is, I think, in line with how many players already thought the feature was working.