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 bac9f0225d)
(cherry picked from commit e7f5c1ba14)
(cherry picked from commit 95d9fe19cf)
(cherry picked from commit 7fc51991e4)
