Remove comment management from the AtomPub service

[snoopdave]

Follows the dev@ discussion with Matt about AtomPub and AI agents: an agent that moderates comments reads text that anyone on the internet can write, and with an admin login the same credentials can also write templates. Until Roller has scoped credentials, comments stay in the web interface.

• Remove CommentCollection, its routes and its service document entry. /{handle}/comments and /{handle}/comment/{id} now answer 404.
• Remove thr:in-reply-to support from AtomReader/AtomWriter; only comments used it.
• Remove the comment tests, doc examples, roller-atompub.rnc elements and the Comments chapter of the AtomPub guide. Regenerated examples drop the unused thr namespace.
• CHANGES.md and the user guide no longer list comment moderation over AtomPub (Roller 7 is unreleased, so no removal note).

ROL-2185's fix (#214) is unaffected.

Testing: full mvn install on JDK 17, 485 tests, 0 failures.

[mraible]

Thanks for doing this so quickly, Dave. I looked through the diff and the only "comment" references left in the AtomPub code are CommentAuthorUrl for category images and the new note in the guide that comments are moderated in the web UI. The ROL-2185 fix and CommentSharedCacheTest are unaffected.

For later: templates are still writable over AtomPub by anyone with an admin login, and an agent that manages a site will usually get the admin's password. Scoped credentials would let people give an agent only what it needs, like app passwords limited to certain collections (entries and media, but not templates) or read-only ones, that can be revoked without changing the account password. That would also be a reasonable way to bring comment moderation back later. I'm happy to write it up as a JIRA if you think it's worth doing.