Allow proper caching with `If-Modified-Since`.

Created by: NotSpecial

There is a subtle difference between no-store and no-cache.

no-store: Never return from cache, always request new. (Especially problematic for images, which are always re-loaded.) This is what we currently have.

no-cache: Don't return from cache by default, but first re-validate with the server, in our case using the If-Modified-Since header, which browsers use automatically, as eve includes Last-Modified headers in all responses. This is what we actually want.

Thus, with this small change, the api should cache in the way we want. Stale results from cache will never be returned without a request, and data is only transmitted if anything has changed.

More info: https://www.keycdn.com/blog/http-cache-headers

Note: Having both no-cache and must-revalidate doesn't actually do anything, as each on it's own would be enough for what we want, but for compatibility its probably best to keep both.

Closes #389 (closed)