Stop redirecting /search.json to the Content API

The new unified search controller has a JSON representation which is 
used to load results in-place on the page. When we remove the 'ui' 
feature-flag, the JSON route for unified search will conflict with 
the existing redirect route.

There are almost zero requests to /search.json in its current form, and 
we recently changed the structure of the API response at the redirect's 
destination without any complaint from users, so I don't think changing 
this route is going to pose a problem.
Make the unified search the default

This changes routes for search so that the unified search action is 
now the default, and requests for search will only hit the previous 
tabbed interface when the 'ui' parameter is 'old'.

This updates the functional tests for the UnifiedSearchController to 
no longer provide the 'ui' parameter, and also prevents the 'ui' param
from leaking out into form requests on subsequent searches.
Merge pull request #575 from alphagov/release-unified-search

Release unified search