Make the most recent flags appear at the top
    
    By inverting the order in which pull_request.flags returns the flags and
    ensure this order is based on the date_created, we ensure that the most
    recently added flags appear at the top when showing them on PRs, rather
    than having them order by update date/time.
    
    Fixes https://pagure.io/pagure/issue/3656
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>