feature: file model, download links & file redirects

[jbmoelker]

todo

• [ ] get custom file URL redirects working, like https://feat-file-model-and-link.head-start.pages.dev/my-files/example.pdf
• [ ] document in docs/routing.md
• [ ] cleanup / refactor earlier attempts

Changes

Associated issue

Resolves #148 Resolves #155

How to test

1. Open preview link
2. Navigate to the Files demo page
3. ...

Checklist

• [ ] I have performed a self-review of my own code
• [ ] I have made sure that my PR is easy to review (not too big, includes comments)
• [ ] I have made updated relevant documentation files (in project README, docs/, etc)
• [ ] I have added a decision log entry if the change affects the architecture or changes a significant technology
• [ ] I have notified a reviewer

[cloudflare-pages[bot]]

Deploying head-start with    Cloudflare Pages

Latest commit:	2f1f2b9
Status:	✅  Deploy successful!
Preview URL:	https://277201ef.head-start.pages.dev
Branch Preview URL:	https://feat-file-model-and-link.head-start.pages.dev

View logs

[jbmoelker]

Split into two PRs? #148 is now working, while #155 is still unresolved. Would require a small CMS change as there's now still the unsupported "custom URL" field in there that's not yet working. And of course, removing unused logic in the code base.