Spectator mode (and maybe auto-AFK)

[BytewaveMLP]

Currently, players are only made spectators if they join in the midst of a game. However, some players may want to sit out a round or two, or record/monitor the action without participating. Players should be able to opt out of playing the next round and be made spectators instead, should they choose.

This could further be extended into adding an auto-AFK system like in TTT, though this is not required.