[Don't merge yet] Release unified search

**Please don't merge just yet. Once reviewed, this needs to wait for a couple more changes (and user research) before we can put this live.**

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).