Provide and run script for making/checking link blocks in CHANGELOG.md

During the last release process I found the process of editing the CHANGELOG.md link blocks rather awkward, and error prone.

It turns out that doing a good enough job of md parsing, for this task, is nontrivial. And the algorithm needed was nontrivial too.

However, I think I like the result. It detected a great number of anomalies in our historical changelog entries.

Also, I just ran this program on tpo/web/blog!310 (merged) and it helped there too.

Fixes #1388 (closed)