Set longer Cache-Control headers

This commit sets the following Cache-Control headers:

- 600 seconds (10 minutes) for things that look like dashboards.
  This means that realtime data may initially be out of date on
  page load, but will be updated immediately after. No other
  modules have a granualarity of less than 1 hour. Most are
  updated daily.
- 7200 seconds (2 hours) on information pages like the homepage,
  services page and about page. These pages are so infrequently
  updated. If we did need to update them urgently, we could flush
  the cache.
Merge pull request #798 from alphagov/longer-cache-headers

Set longer Cache-Control headers