add govuk-frontend-toolkit dependency
use external-link mixins for external link styling
Correctly apply 'highlighted' class when hovering over annotations.

@pbadenski
@abersager
use external-link icon mixin from govuk-frontend-toolkit for content explorer.

@pbadenski
@abersager
Merge pull request #11 from alphagov/fix-external-link-icon

Fix external link icon