Update toolkit to bring in device-pixel-mixin
Add new external link style variations from toolkit

Includes fixes for toolkit update & some extra external link
styles
Add new external link CSS from toolkit
Update toolkit to bring in external link mixins
Changed Classifiying goods and finding commodity codes link to Classification guides
Merge pull request #69 from alphagov/classification-guides-link

Changed wording of Classifiying goods and finding commodity codes link
Merge pull request #68 from alphagov/external-links-update

External links update