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
Yes, the reason is to make this function also accommodate use cases when there is a `Message` object in hand instead of a message code. see https://github.com/Wikia/app/pull/5804/files#diff-b09b82a91e5b339b04d7f97b02db3e85R233 for instance.