Show the changelog in the web UI, with a "what's new" popup after an update #401

Follow-up from #388 (thread).

The Message model / messages feature has never been used. As far as I can tell, the only way to create a message is a data migration, and no migration does that yet. But it's the one channel for reaching users who aren't on Discord. A better use for it would be the release changelog:

• Add a CHANGELOG.md to the repo, in the format Home Assistant add-ons use, so it also shows on the add-on's page in HA (add-on presentation docs).
• Bundle the changelog into the manager image at build time and show it in the web UI.
• On the first visit after an update, show a popup with the entries since the last version the user saw. This could reuse the existing unread flag on messages.

Open question: how the changelog gets into the manager build, e.g. copied into the image at Docker build time, or generated from release notes.