Add a consumer for updating AMI link page
    
    This page won't be properly populated when the event is first
    created, as that happens as soon as the compose is complete but
    the AMIs are published over the next hour or two. We do not get
    a signal when *all* of the AMIs are published, unfortunately, so
    all we can do is regenerate the page *each time* a relevant AMI
    is published. That's what this consumer does: whenever an 'AMI
    published' message is received, it checks for a validation event
    related to that compose, and if there is one, it updates its AMI
    page.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>