drop 'nightly', handle fedfind 2 and pungi 4 nightlies
    
    First steps in adapting relval to fedfind 2 and Pungi 4. We
    drop the 'nightly' subcommand entirely because all that woo
    is gonna move into a new dedicated fedmsg consumer for creating
    events when new composes appear. So now we just use 'compose'
    for creating nightly events as well, when/if we ever need to
    do it manually.
    
    We also allow creating events by passing a Pungi compose URL
    rather than release/milestone/compose, using the wikitcms
    get_validation_event_url code.
    
    size_check gets adapted to fedfind 2.x, the conversion wasn't
    too painful happily enough.