fix generated source URL on rendered files (#26364)

fix generated source URL on rendered files (#26364)

- The permalink and 'Reference in New issue' URL of an renderable file
(those where you can see the source and a rendered version of it, such
as markdown) doesn't contain `?display=source`. This leads the issue
that the URL doesn't have any effect, as by default the rendered version
is shown and thus not the source.
- Add `?display=source` to the permalink URL and to 'Reference in New
Issue' if it's renderable file.
- Add integration testing.

Refs: https://codeberg.org/forgejo/forgejo/pulls/1088

Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-authored-by: Giteabot <teabot@gitea.io>