Addons + Proxito: return `X-RTD-Resolver-Filename` and inject via CF (#11100)
* Development: use `wrangler` locally (update NGINX/Dockerfile config)
Related https://github.com/readthedocs/addons/issues/217
* Add CF Worker .js script
* Proxy JS files to Addons GitHub's repository
* Point common to wrangler branch
* Addons + Proxito: return `X-RTD-Resolver-Filename` and inject via CF
Return `X-RTD-Resolver-Filename` from El Proxito and inject it as HTML `meta`
tag from Cloudflare Worker.
Required by https://github.com/readthedocs/addons/issues/211
* Feedback from review
* Validate the header value before injecting it
* Initialize `request.unresolved_url` as `None`
* Add test for longer filename
* Inject the `unresolved_url` on 404 pages