Feature/forum permissions

Note that this is in no way a sustainable solution. Forums aren't tied
to games in any way, and permission is based entirely on the string name
of the Forum. If we can inherit from and register a substitute for the
forum class (with actual game logic baked in), I'll be happy to throw
this out.

This commit also creates a `nearest_game` function which gets the latest
game if no imminent game exists; this will allow people to see stats and
player lists after this game is over :whale:

This fixes #150.