allow notifications icon to go grey when all messages have been seen - fixes https://github.com/HabitRPG/habitrpg/issues/3270

@lefnire : migrations/20140823_remove_undefined_and_false_notifications.js needs to be run when this goes live.

Explanation: https://github.com/HabitRPG/habitrpg/issues/3270#issuecomment-51696497

This also prevents the "undefined" guild from reappearing (https://github.com/HabitRPG/habitrpg/issues/3270#issuecomment-41032939) because that was happening when ````$http.post('/api/v2/groups/'+gid+'/chat/seen');```` was called with no gid (i.e., every time you visited the party page). Now that the api is deleting seen message data instead of setting it to false, it never incorrectly creates the undefined entry.
It did work when I tested it (the entry in newMessages for that guild was removed). [Docs](http://docs.mongodb.org/manual/reference/operator/update/unset/) indicate that even a false value is suitable (which I do think is weird).