Set cache-control headers for responses when running in prod

[edmondchuc]

Set cache-control headers to enable browser and gateway caching.

Most things will benefit from caching. Users in browsers often go to the same pages, if these are cached in the browser, it's instant.

Gateway caching has huge benefits when it comes to clients scraping the data in a Linked Data follow-your-nose manner. Good to set it with stale-while-revalidate strategy.

We may also utilise ETags for more dynamic things, especially when later we have write capability in Prez.