Fixed #13723 -- Improved the legibility of hyperlinks included in admin...

Fixed #13723 -- Improved the legibility of hyperlinks included in admin validation error messages. Thanks to Sebastian Noack for the suggestion and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13659 bcc190cf-cafb-0310-a4f2-bffc1f526a37