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
Could you add `https://developers.facebook.com/docs/facebook-login/manually-build-a-login-flow/v2.1#deauth-callback` here too