Better caching directives

[flakey5]

We're serving mostly static and immutable content, but only caching it for 1 or 4 hours (depends on cache type) for successful responses.

I think we should change this so that content that doesn't change (i.e. release assets) are cached for a lot longer than assets that do change (i.e. some directories, index.json).

Specifically, I think they should be something like these:

• for immutable content (release assets, metrics, ...), public, immutable, max-age=31536000, s-maxage=31536000
• for the things that do change (index.json, directory listings, ...), public, max-age=86400, s-maxage=86400, must-revalidate

[flakey5]

cc @ovflowd