• Fixes https://github.com/sourcegraph/sourcegraph/issues/43839

I've done some digging, and I don't see a truly elegant solution that I could deliver before tomorrow's branch cut.
The solution in this PR is not elegant, but it solves the current customer issue, and I find it unlikely that it'd break other cases.

My findings:

• The repositoryPathPattern is not available on the front end (for a reason).
  • Only the modified repo URLs are available, that is, the results of applying the repositoryPathPattern on the original URLs.
  • repositoryPathPattern is a part of the code host connection settings (The config visible at /site-admin/external-services/{id}), and the whole thing is not accessible to the front end. This is expected because this raw information should probably be available for admins.
  • Without repositoryPathPattern, we can't reliably determine how many pieces we should cut off from the URL.
• We could move the logic to the back end, and have the back end return an alternative path made for Open in Editor. This would probably involve introducing a new setting to the code host connection config (for each code host type), to set the number of pieces to cut off explicitly. Then the front-end logic would have no logic; it'd just use the URL provided.
  • I'm not confident that this would be a user-friendly solution; it's a bit too many settings, and I'm still wondering whether the cutoff point could be reliably calculated on the back end.

Test plan

I've added a new test that covers this case; the old test and the new one are all passing, so I think this should be fine.

App preview:

• Web

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