Use site credentials in reconciler and changeset syncer

Created by: eseliger

This PR introduces the site_credentials table to the mechanisms for determining an authenticator to both the changeset syncer and reconciler. Also, we now start telling site-admins too when there are no credentials configured for a code host, so they can start finding out about site credentials 😎. The final migration plan and deprecation will come in a separate PR, and this doesn't change any previous behavior, if no site-credentials are configured.