[games] Add 'game_over' API function for deciding a winner

Updates games to make use of this new function, and moves functionality
preserving the game board for players after a game has ended from MnkGame to
the general base template.
[games] Make game state variables into instance variables

Solves #25
[games] Use 6-char hex colours

Fixes #17
Merge branch 'master' of https://github.com/smab/playhouse-web