Threaded comments

Allow users to reply to comments by clicking a new "Reply" button.
There will only be one level of reply to avoid query complexity and
recursive indentation hell. There are two UI clues that a comment is a
reply: indentation and "replied" rather than "commented."

The order for top-level comments is "latest first" while the order for
replies to a top-level comment is the more natural "earliest first."

Implementation details follow.

A new property on Activity named `in_reply_to` tracks comment replies.
On the relevant pages and snippets, use several new Alpine `x-data`
variables to track whether a comment is a reply or not. There are some
assumptions made about IDs and these variables between HTML pages and
partial pages.

In order for migrations to succeed on a fresh database, for example a
database migrated during automated tests, an older migration needed to
be nerfed. It had made use of an Activity object during a migration.

Issue ots/clients/ardc/hypha-tracker#356: Threaded comments