Create new template for legacy blog/newsroom items

[csebianlander]

This PR creates an indpendent HTML template for Legacy Blog and Legacy Newsroom page types, to allow the template-level addition of an "archived content" banner.

That banner currently exists in rough form with a placeholder URL built out of a (not yet functional) Archive-It URL string and a generated canonical URL for the page rendered by the template. It's not functional but you can still see how it will work in practice once a proper organization account and crawl collection has been established in Archive-It.

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How to test this PR

1. localhost
2. Create or edit/view a Legacy Blog or Newsroom page

Screenshots

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

[anselmbradford]

Some updates:

• Moved the legacy banner to the correct block.
• Added a banner template, and made the legacy banner use it.
• Made the FIG use the banner template and removed the period from the end of the FIG link URL.
• Made the beta banner use the banner template.