Make homepage editable in Wagtail

[csebianlander]

This PR reworks our homepage, currently hard-coded, into editable fields in Wagtail

It is currently incomplete and in very draft form as I start hacking at the existing code and trying to build some bespoke field panels for use on the home page.

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How to test this PR

1. localhost
2. View /, both in its front-end rendering and back-end forms

Notes and todos

• This will probably need to be a multi-phase rollout to ensure we don't have a period of displaying a broken/empty homepage. I'm thinking one PR with all the back-end changes but which preserves the current hard-coded html pages, and then a second PR which swaps those out for the correct template files.

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

[csebianlander]

The relaunch of the homepage almost certainly will happen before this PR goes any further, but work to make the homepage properly editable should probably happen at some point soon after that relaunch.