This adds links to download a directory as archive. This can be useful if you e.g. just want to download a assets directory instead of the full source tree. The logic already exists in the backend, so only the frontend had been changed.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/4736
Reviewed-by: 0ko <0ko@noreply.codeberg.org>
Co-authored-by: JakobDev <jakobdev@gmx.de>
Co-committed-by: JakobDev <jakobdev@gmx.de>