Respect cache header from custom endpoints

I might have overlooked something, but it seems like `Cache-Control` headers are not properly respected for requests to custom endpoints, are they?

Requesting a badge twice using a custom endpoint, which returns `Cache-Control: no-cache`, resulted in only one request to that certain endpoint in my experiments.

How is does Shields implement caching in general? What resource are cached and for how long?

Reproduce the custom-endpoint request twice with a Cache-Control: no-cache response, then trace the repository's general caching implementation and the custom-endpoint path. Done means the observed request behavior matches the header semantics and the applicable cached resources and durations are documented or otherwise verified.

Written by the indexing model from the issue text.