Add link to National Archives from no results page

People are trying to search for content on GOV.UK for publications which
were published pre-May 2010. These documents were mostly not migrated
over to GOV.UK however, they can still be found on The National Archive.
This will provide people another avenue to search down if they can't
find the document they are looking for.
Auto fill search term on TNA page with search term
Merge pull request #391 from alphagov/tna-link

Add link to National Archives from no results page