Server should fork/edit Gists on behalf of disconnected users

[LearningNerd]

As outlined in issue #20, if a player disconnects before the end of their turn, the Gist should be forked/edited on their behalf so as not to misattribute their edits to the next player. But here's the problem: the client can't fork or edit the Gist after they've disconnected from the app! So the server will have to handle these edge cases.

Tasks for this issue:

• Save every player's GitHub access token on the server as part of the game state.
• On disconnect, the server should check if the current player disconnected before the end of their turn; if so, the server should fork/edit the Gist on the player's behalf before restarting the turn timer and passing control to the next player.