Declare a cache policy for downstream users

[chris48s]

• Serve cache-control headers explaining how long content can be cached for (gut instinct is 30 mins but TBC)
• Document it

NB: For this purpose, I think the cache-control headers need to be set up in CloudFront. I think if we serve them from the django app, that actually tells CloudFront how long to cache our content. What we actually want to do here is tell our downstream consumers how long they can cache for.