More Match Quitting Flexibility

Combining three older issues that should likely be handled together - we should make it easier for players to quit and/or leave a match they don't wish to be in, since holding them hostage to something they aren't contributing to is a waste of everybody's time.

- [ ] Per #1010 and #3839, in a multiplayer game, a player should be able to concede and/or quit and *leave the table* without it hanging around in the background hogging performance and triggering occasional UI displays over other games. Doing so should result in them no longer being rejoined to that table when leaving and returning to Xmage.

- [ ] Per #272, players should be able to concede and/or quit during sideboarding, without having to submit, wait for the match to start, and then concede.