Add link to national archives if no results

People trying to find documents might not know that older content is not
on GOV.UK we should help those people find the content they need.
Update text to use the same text as main search
Add keys to all locale files
Merge pull request #645 from alphagov/nta-link

Add link to national archives if no results
https://www.pivotaltracker.com/story/show/51553001