Jumoo Releases: activity page for commits since last release

Repo: Jumoo/releases

Added an activity.html page listing, per package, commits since its last tagged GitHub release — a way to spot packages overdue a release.
Fixed a subtlety: releases aren't always cut from a repo's default branch (e.g. Xliff Connector defaults to v17/main but shipped v18.0.1 off v18/main) — the fetch script now resolves which mainline branch a release's tag actually sits on and diffs against that, instead of the repo default.
Flags packages with no tagged GitHub releases separately as "no tagged releases".