WIP: Explore replacing RichText footnotes on the server

[dmsnell]

Given a post_content with <data value="core/footnote">[Some reference]</data> this replaces on render that content with a link to a later footnote at the end of the post, then appends a list of footnotes at the end of the post.

Borrows code directly from #47682

This is silly code, please DO NOT MERGE.

[github-actions]

Flaky tests detected in 222db00.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/4679919331
📝 Reported issues:

• [Flaky Test] should insert elements from multiple completers in a single block #47916 in /test/e2e/specs/editor/various/autocomplete-and-mentions.spec.js

[ellatrix]

Cherry picked your commit into #49797

[dmsnell]

@ellatrix I'm closing this because I think it's no longer needed