Add HTTP cache headers to release APIs

[preaction]

We should be able to improve our users' experience with this API by adding appropriate HTTP caching headers to this API:

• Add an accurate Last-Modified time to the release API
  • This time should be the last updated time of the last report that was made part of this data
  • This can be found by joining the report table via the release data's guid column. See CPAN::Testers::Schema::ResultSet::Release->since for an example.
• Respond to If-Modified-Since request header
  • Check the last modified time we would send out. If the If-Modified-Since header is greater than the Last-Modified time, send back 304 Not Modified instead of the data
• Add Cache-control headers to the response
  • This data only updates every 60 minutes, so tell users they can cache it for 60 minutes (max-age=3600)