#37 Add a configurable deadline for scheduled game cancellation

[Alf Mikula]

Allow users to specify their own preferred deadline for canceling a scheduled game. ie, if they specify a deadline of 4 hours before the game, then once that time window passes, their opponent may not cancel the game, but instead will be given the option of forfeiting the game.

[Alf Mikula]

Logged In: YES
user_id=1882138
Originator: YES

This cancellation window should also be considered the amount of time before the scheduled game time that a game offer must be accepted. ie, an offered game with a cancellation window of 4 hours should expire 4 hours before the start time of the game.