Add IF Archive and subdomains to direct domains list

[dfabulich]

Apropos #149

1. IF Archive has CORS headers; there's no need to use the proxy in this case. We could whitelist some domains and list them as not requiring a proxy.
2. Instead of (or as well as) whitelisting, on the client-side, we could optimistically try to fetch a files directly; fall back to the proxy only if the fetch request fails.
3. Update the Cache-Control header to use stale-while-revalidate https://web.dev/articles/stale-while-revalidate perhaps something like: Cache-Control: max-age=60, stale-while-revalidate=604800.
   Unfortunately, Cloudflare doesn't support stale-while-revalidate yet; they say it's coming later this year.

This implements option 1, but I'm not sure we should do it this way; perhaps we should do option 2 instead, in PR #150.