Add Google Analytics tracking code to Bouncer's 404 and 410 pages

- This is meant to provide richer data on archive and error pages so
  that we can see what users do next and where they came from. It is
  intended to help investigate strange-looking traffic spikes.
- This is only included in non-development environments.
Update the Google Analytics tracking code

- This is following on from Robin's meetings with Ashraf. It's a
  different code specific to Bouncer for our purposes.
Merge pull request #105 from alphagov/analytics_on_404_and_410_pages

Add Google Analytics to 404 and 410 pages