Continues the work done on
https://github.com/discourse/discourse/pull/30815.
Adds a `footnote` node, parser, `^[inline]` input rule, toolbar button
item, and serializer.
Also adds a NodeView with an internal ProseMirror editor to edit the
footnote content.
In a previous PR, I introduced this system spec that checks that a sidebar link is auto-generated for certain plugins.
This causes problems, because the core test suite can be run with plugins either enabled or disabled, causing flaky tests.
Using SiteSetting.queue_jobs= to configure job asynchronicity was deprecated here four years ago and marked for removal in version 2.9.0. This PR removes the fallback method we kept since then. The method was there because it was still being used in a bunch of plugin tests (now fixed.)
