Add overrideable custom user selectors for external services.

In general this adds the possibility to search for existing users when logging in from an external service. Right now this is only possible when a connection already exists.

Example use case: Connect facebook account to an existing app user when they have the same email address.

This is useful when you add facebook login after your app was already released and your existing users (who are not loggedin) start using facebook-login (without first connecting their facebook account).