Set cache-control headers to public

Edge-case: when returning a draft item, set them to max-age=0, private
Add cache-control headers to finder endpoints

Add these headers to the /business-support-schemes and /licenses
endpoints.
Add cache headers to search endpoint
Set cache-control header for /local_authorities endpoints
Merge pull request #137 from alphagov/fix_cache_control

Improve setting of cache-control headers.