Add Google Analytics to 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 Google
  Analytics code is specific to Bouncer for our tracking purposes.
- This is only included in non-development environments.

(Testing this in development required me to remove the "not in development environments" restriction, as `$ RACK_ENV=production bowl bouncer` didn't actually change my environment when I was trying to test it. However, doing this for the development environment proved that it works fine - we have data in Google Analytics for these pages.)