Release unified search

https://www.pivotaltracker.com/story/show/66674874

This makes the necessary changes to the Frontend app to switch the default search on GOV.UK from the existing tabbed view to the new unified view.

This also changes the behaviour of `/search.json`. It no longer redirects to `/api/search.json`, instead routing to the unified search action so that it can respond to live search requests. I don't believe this change will negatively impact any users (detailed more in the message of the relevant commit).