This implements "repository flags", a way for instance administrators to
assign custom flags to repositories. The idea is that custom templates
can look at these flags, and display banners based on them, Forgejo does
not provide anything built on top of it, just the foundation. The
feature is optional, and disabled by default. To enable it, set
`[repository].ENABLE_FLAGS = true`.
On the UI side, instance administrators will see a new "Manage flags"
tab on repositories, and a list of enabled tags (if any) on the
repository home page. The "Manage flags" page allows them to remove
existing flags, or add any new ones that are listed in
`[repository].SETTABLE_FLAGS`.
The model does not enforce that only the `SETTABLE_FLAGS` are present.
If the setting is changed, old flags may remain present in the database,
and anything that uses them, will still work. The repository flag
management page will allow an instance administrator to remove them, but
not set them, once removed.
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>
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