DownloadPage: downweight images with type "live-osbuild"
    
    This type denotes 'experimental' osbuild-built live images; if
    and when we make osbuild the "official" builder of lives, they
    will have the normal 'live' type, not 'live-osbuild'. We should
    bury these images down the download table, not put them right
    at the top as we do currently.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>