- 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