Validate smart quotes as normal quotes in links

www.agileplannerapp.com/boards/173808/cards/8852

the `LinkValidator` regex doesn't validate correctly
when the link contains smart quotes. corrected that
so that we can update travel advice publisher to use
the latest version of govuk_content_models.
Merge pull request #267 from alphagov/validate-smart-quotes-in-links-as-normal-quotes

Validate smart quotes as normal quotes in links