UC-116 Facebook Client Service & improved user-mapping related error handling

This adds a Facebook Client Service to unify the way user mappings are created. This allows for all controller logic to go through the service in order to create a user mapping. This service is the front for communicating with FacebookMapModel (partially for now). We can then refactor more operations later into the service to avoid duplicate logic in multiple places.
This also simplifies error handling; now that we have more error cases to cover.

* Also excludes UserSignup special page from return-to logic, so that user does not land on it after logging in.

https://wikia-inc.atlassian.net/browse/UC-116
@armonr and I already discussed this. There's actually a more complete list of pages on the back end [here](https://github.com/Wikia/app/blob/dev/extensions/wikia/FacebookClient/FacebookClient.class.php#L376). Since I believe this JS includes a server request anyway, it would be great to keep that logic all in the php and just return the correct `returnto` location to the JS. That's the model I had set up for signup so we should keep that going for login.