koji hub stg: Set up policy for CoreOS continuous builds
    
    Allow users with coreos-continuous permission to add/block/unblock
    packages in *-coreos-continuous tags.
    
    For more details, see:
     * https://meetbot.fedoraproject.org/teams/releng/releng.2019-03-06-16.00.log.html
     * https://pagure.io/releng/issue/8165