Expose the repository flags feature over the API, so the flags can be
managed by a site administrator without using the web API.
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>
(cherry picked from commit bac9f0225d47e159afa90e5bbea9562cbc860dae)
(cherry picked from commit e7f5c1ba141ac7f8c7834b5048d0ffd3ce50900b)
(cherry picked from commit 95d9fe19cf3ed5787855ac2a442d29104498aa36)
(cherry picked from commit 7fc51991e405ea8d44fd6b4b4de13ad65da63ae7)
