Implement drawing

[spamguy]

At any point during a game, if all players agree to the same draw condition, the game should end with a status of 2, and all players in the draw condition should be awarded a win, or a partial win, depending on the algorithm used.

The feature should be available from the 'Actions' tab.

Drawing is less likely in gunboat games, so it's probably OK as a > 0.1 feature for now.