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
These 3 special pages that we want to redirect from are repeated 3 times. Here, and then again in UserLoginFacebook.wikiamobile.js and topbar.js. Maybe there's some way we could consolidate them to reduce repeating code? I'm not 100% sure how, maybe as a class constant somewhere?

@armonr, @lizlux any ideas where that could go?