semi-automated addition of new "forges"

Use case: extend archive coverage to a specific GitLab instance (specified by URL) as seamlessly as possible.

(The obvious generalization is replacing GitLab with any kind of supported listable source code origin out there, e.g., another Debian-like distro, another PyPI instance, etc.)

We currently can, with a single command (1) add an entry to the list of "forges" being listed. What we lack to implement the "as seamlessly as possible" part above is:

(2) immediately do the full listing, with high scheduling priority (3) once (2) is done, immediately load all listed origins, with high scheduling priority (4) bonus point: notify the user once (3) is done

As another bonus point, having the above doable with a single CLI command would be great.

Once we have this, it will be the obvious building block of a "save forge now" user-visible functionality in the Web UI (which will be tracked in a separate task).

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Migrated from T1537 (view on Phabricator)