Fix repository mirror updates for new imports stuck in started

We were calling RepositoryUpdateMirrorWorker that sets the import status to finished and then add the import job. Then update_mirror didn't do anything but set the import_status to started. This should be now getting called in the right order...

Closes https://gitlab.com/gitlab-com/operations/issues/287 and https://gitlab.com/gitlab-org/gitlab-ce/issues/17747