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
This may be better off out here so that this exception can specifically be caught. I originally had the service method throw this exception, but at least the service and its consumers still should be able to access some FB-client specific exception(s). The subclasses, however, are used internally by `FacebookMapModel`, so they don't need to be exposed.