Make the main links on the "save code now" request list a link to the archived copy

I'm always confused by the external links on the save code now summary page (https://archive.softwareheritage.org/save/#requests).

I think it would make more sense if the main links pointed to the latest snapshot on swh (when it exists), and have the link to the original repository in a separate column with an explicit external pointer (like we do on the header of archived repositories).

(This quirk doesn't seem to be handled by #2324 (closed) / #2325 (closed), so I figured I'd submit it separately.)

---

Migrated from T2405 (view on Phabricator)