Style corp information page language links

This extracts the styling for .available-languages into
a css helper, taking it out of the heading scope.
Merge pull request #1588 from alphagov/fix-translation-links-on-corp-info-page

Style corp information page language links
Handle Invalid URIs in LinksChecker

URI.parse will fall over if the provided URI is invalid. This adds error
handling to LinksChecker so badly formed URIs do not cause the checker
to fall over.
Merge pull request #1591 from alphagov/handle-bad-uris

Handle Invalid URIs in LinksChecker