Migrate incorrect Via 3 PDF document URLs

As a result of https://github.com/hypothesis/via3/issues/372 all existing annotations created on PDFs using Via3 before 2021-04-06 have an incorrect `target_uri` field for the annotation. Instead of being the original URL of the PDF it is Via3 URL which proxies the PDF. In other words rather than `https://example.com/file.pdf` the `target_uri` is `https://via3.hypothes.is/proxy/static/https://example.com/file.pdf` or similar. We went through several different domains while working on a replacement for Via, including at least via3.hypothes.is and via4.hypothes.is.

This issue was being fixed by https://github.com/hypothesis/via3/pull/427 but existing annotations will need to be updated via a data migration or some other mechanism.

Fixing this issue will resolve a problem where the document details show up incorrectly in the Notebook and activity pages (https://hypothes.is/search) for annotations created with Via 3:

Be aware that it is not just the `annotation.target_uri` field which matters here. There are corresponding fields in the `document_uri` and `document` tables as well.