Ability to draw a game

[zac-keysys]

Either the 3-fold repetition rule or a manual draw command that can be entered to end games without one player having to resign.

[cjsaylor]

@zac-keysys The library that the chess bot uses behind the scenes already has the ability to recognize specific draw conditions, we would just need to check it after a move is made.

A nice interface for this may be to include some text about the draw conditions detected and then offer a button whose action would be to offer a draw. This would then flow into a call/response from the opponent to end the game as a draw.

This could be tied in with #25.