Use gitlab-friendly URLs when formatting changelogs as HTML
Our old https://bugs.torproject.org/nnnn URLs only work for bugs numbered before 40000. Newer gitlab bugs need to have specific projects mentioned.
This patch assumes that bugs are in tpo/core/tor by default, but allows us to refer to several other projects by saying e.g. "chutney#nnnnn" if we want.
Our format_changelog script is not distributed, so no changes file is necessary.
Closes #40051 (closed).