* Looks for heading IDs matching "h-<heading text>-%" that once
existed on the target page.
* For such IDs, finds where those items currently exist,
presumably in an archive.
Pros:
* Doesn't need to know anything about the local wiki's archiving
conventions, so can be deployed universally.
Cons:
* ID conflicts will return matches in unrelated archives, e.g.
MassMessages.
Bug: T349653
Change-Id: Ie94efd0503e9f4689d3421babe445f9f4e2b4fb7