Emit 'location' key in fedmsgs, pointing to compose URL
    
    For fedmsg_meta we have to provide a 'link'. At present we do
    this with a rather large and icky pile of conditionals which
    doesn't work properly. It's much simpler to figure out the URL
    in the compose script and emit it in the fedmsg, then meta can
    just...use it. For older fedmsgs we'll just point to the base
    kojipkgs location for the link.
    
    For synced composes, we use the synced URL on dl (this matches
    what the meta stuff tried to do before). For non-synced composes
    we just use the kojipkgs location.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>