removes User._id and sets up _playerId - changes when the user logs out
so that the ui can recognise that the user isn't a player in any games
they're viewing anymore (Game will be changed to check the playerId instead
of the id/username)
changes Game.getUserColour to use the user's playerId
changes Game to use a Promisor
corrects Premove creation in Game /premove handler
updates ChallengeGraph to use owner.name instead of username
updates Game, GamePage and Chat to use new Player interface (name instead of username)