Fix back button behavior in modern browsers

The problem here was we were storing the page state before we updated
it. So you would get a stale cache when you used the back button.

Added a test which highlighted non-determanistic behavior due to the way
we were 'mocking' in a different test. Made sure that those tests reset
the global objects back to how they should be when the test exited.

https://www.pivotaltracker.com/story/show/46889523
Rake task for describing filters from a list of urls

This is intended for use with info4local import.

Note that the topics param could be split by semicolon, but in the real
urls that's not a valid url. This is an artefact of the way the data is
exported from the govuk_delivery system.