@extend %clearfix instead of mixin

The clearfix mixin is really long, and is well-suited to being a mixin, so this pull request takes care of the sassquatch side of the conversion. It looks okay in style guide pages, but will need to do a bit more regression testing along with this bug: https://bugz.meetup.com/show_bug.cgi?id=38072

Merge will probably need to be managed manually (not on GH), with a release folded into bug 38072.