UC-86 Adding a callback handler for facebook's disconnect notifictaion

Facebook provides a way to notify an app owner when a user disconnects the app from their account.  Setup a handler for this and delete the FB <=> Wikia mapping when we receive it.

https://wikia-inc.atlassian.net/browse/UC-86
It does have a logger call (see next line); looks like I just forgot to delete this one!