Loose teams

[rossjrw]

Sorting players by alphabet was always a stupid idea. As of right now there's 23 unsuccessful moves to 60 total moves, indicating that it is not intuitive and overall a bad idea.

Instead, players should not be grouped into a team until they have played a game. At that point they should be locked into that team until the game ends.

• Team check can return null
• Handle null case when team check is made
• Use game state to derive teams instead of the alphabet
• Update prose everywhere to reflect new selection system
• Update messages e.g. in new issues to reflect team joining/team participation
• Add a 'who's on each team' to the README

Closes #53.