Add caching headers to existing Kolibri public APIs

Background

We're adding new V2 APIs from Kolibri to Studio, and we're adding Cloudflare page rules to make sure that we can obtain optimal caching of the API responses

Desired behavior

The existing /api/public endpoints:

• should return an appropriate Last-modified and support handling the If-modified-since header
• should return cache control headers: Cache-control: public, stale-while-revalidate=100, max-age=300

Current behavior

These endpoints do not return or support caching

Value add

Better edge caching through Cloudflare, meaning better worldwide browsing of public channels through Kolibri.