Improve the efficiency of the service with caching

We can improve the speed and cost efficiency by using caching in certain circumstances:

* Resources which are flagged by our internal process can be cached for a long time
* Resources which are flagged by external services can be cached for a shorter time
* Resources which are not flagged by anything **cannot be cached** as they could prove bad at any time (This could be cached for a short time, at the cost of a delay in responsiveness)

### Implementation notes

Depending on whether we end up protecting our end-points with a blanket block, or a more tailored approach in https://github.com/hypothesis/checkmate/issues/9, Cloudflare might not be in the mix from a point of view of caching.

In that case we would probably have to fallback to local NGINX caches and client cache, which isn't as good. And as our services are the clients, they won't cache.