Set a cookie based on the position of link clicked

So that we can find out where users are clicking on search results we
are going to set a cookie containing the position of the link. We will
then save the position information to Analytics on the next page.

This is done via a cookie and on the future page so we don't block
leaving the page while we call off to analytics.
Make cookie generic so it can be use elsewhere
Merge pull request #535 from alphagov/track-search-clicks

Set a cookie based on the position of link clicked