Disconnect from Firebase on logout

[dborkan]

Currently on logout, we don't disconnect from the Firebase server.

I've tried calling Firebase.goOffline() in logout, however a side effect of this is that future login attempts for the same social provider fail (authWithOAuthToken never invokes it's callback). We should figure out why this had been happening, and call Firebase.goOffline() in order to free up Firebase connections so we don't hit our quota.