Add an alembic revision dropping the pull_request_check constraints
    
    That constraints is making sure the pull-request is either coming from a
    fork or a remote git repository.
    Except that when we delete the fork, we still want to keep the
    pull-request records, so we can no longer enforce that constraint.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>