Add models to tag the compose and set user-based tag/untag permissions.
    
    - Adds `models.Tag` representing the compose tag.
    - Adds relation between Compose-Tag to allow tagging composes.
    - Adds "tagger" and "untagger" relations between Tag-User
      to allow configure who can tag/untag compose using particular
      tag.
    
    Merges: https://pagure.io/cts/pull-request/5
    Signed-off-by: Jan Kaluza <jkaluza@redhat.com>