Add wagtail-footnotes

[willbarton]

This change adds the wagtail-footnotes library and enables footnotes for all rich text fields.

It does not render footnotes that have been added.

This is for experimenting with the content manager side of footnotes for now.

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Additions

Removals

Changes

How to test this PR

Screenshots

Notes and todos

Checklist

• PR has an informative and human-readable title
  • PR titles are used to generate the change log in releases; good ones make that easier to scan.
  • Consider prefixing, e.g., "Mega Menu: fix layout bug", or "Docs: Update Docker installation instructions".
• Changes are limited to a single goal (no scope creep)
• Code follows the standards laid out in the CFPB development guidelines
• Future todos are captured in comments and/or tickets
• Project documentation has been updated, potentially one or more of:
  • This repo’s docs (edit the files in the /docs folder) – for basic, close-to-the-code docs on working with this repo
  • CFGOV/platform wiki on GHE – for internal CFPB developer guidance
  • CFPB/hubcap wiki on GHE – for internal CFPB design and content guidance

Front-end testing

Browser testing

Check the current browser support cutoff list](https://cfpb.github.io/consumerfinance.gov/browser-support#current-browser-support-metrics) for browsers that are advisable
to prioritize for testing.

Accessibility

• Keyboard friendly (navigable with tab, space, enter, arrow keys, etc.)
• Screen reader friendly
• Does not introduce new errors or warnings in WAVE

Other

• Is useable without CSS
• Is useable without JS
• Does not introduce new lint warnings
• Flexible from small to large screens