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.
