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></pingou@pingoured.fr>