add PostRelease class for post-release nightlies
    
    maxamillion pointed out the existence of post-release nightly
    builds, which currently only build Cloud images, and live in
    stage/ alongside TCs/RCs. This class handles them reasonably
    well, with the significant drawback that we can't 'wait' for
    them as there's no 'canary' file that shows up when the compose
    is complete, like there is for Rawhide and Branched nightlies.
    
    We also handle it in get_release(), which makes it even more
    of a maze, but oh well. We replace the rsync scrape to find
    current/next release with the new get_current_release() (see
    previous commit) while we're at it.
    
    expected_images is more or less a placeholder at this point,
    as it needs re-designing for the new 'deployment' property,
    coming up in next commit.