Cache API responses

Cache API responses, both on Varnish and in the user's browser.
The imageinfo request is not cached, since that would make it very
hard to test metadata template edits. Everything else is cached for
one day.

Change-Id: I9149cf40d4448a424073eefd1eb442c70c977687
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/435
Merge "Cache API responses"