Better support for browsers with JS disabled

Breaking this discussion out of !1073 (closed).

IMO, swh-web depends on client-side Javascript more than it needs to:

1. The origin search does not work at all
2. The "Save Code Now" form is a noop without it
3. The "Add Forge Now" form works but shows an obscure error (#4053)
4. Tabs in "Save Code Now" and "Add Forge Now" don't work
5. We had to repeatedly fix history issues with these tabs, even with JS enabled (#4050 (closed), rDWAPPS39bab96e, an old diff I was working on that needed to add a tab somewhere, ...)

All these could be replaced by normal HTML pages and forms; this would allow them to work everywhere, and be less brittle.

(Note that I'm not advocating for complete removal of client-side Javascript; it is fine to use it for Datatables, lazy loading of origin statuses, lazy loading of status.softwareheritage.org, lazy loading of counters, querying GitHub's API, ...)

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Migrated from T4054 (view on Phabricator)