Make external statistics links clearer, mirroring the style used in search results
Fix external publication cucumber test

Previous test depended on template copy, refactor it A) pass and B) test what we now care about:
* There is a link to the external publication itself
* The link is also marked as rel=external, which will visually highlight the externalness to the user
* Ensure the full external URL is visible to the user, somewhere, to set an expectation of where they're going
Merge pull request #1463 from alphagov/external-statistics-link-styling

Make external statistics links clearer