Update 404 page content

- New title and body links.
- Adjust test for output.

https://www.pivotaltracker.com/story/show/71945442
Do we need the `https://www.gov.uk` here? We should be able to use `/search` and `/`, which we do in other error pages, eg https://github.com/alphagov/static/blob/master/app/views/root/406.html.erb