add koji_done and pungi_done properties to Releases
    
    These can be used to check if a compose is done. Obviously this
    is dumb and we should be using fedmsg, but right now there is
    no fedmsg sent after the Koji builds for a nightly compose are
    done: there's a signal when they're kicked off, but not when
    they're done. So you get this hack instead. Merry Christmas.
    
    Non-Nightly Releases implement the properties so users don't
    have to check, but they just assume the tasks are done so long
    as the release 'exists'. This isn't *quiiiiiiiiiite* entirely
    true, but good enough for government work.