Switch to &:extend(.clearfix all) for clearfix mixin

Original discussion:
https://github.com/less/less.js/issues/1437#issuecomment-21383639.

Since we’re switching to `grunt-contrib-less`, we can take advantage of
newer LESS features than what RECESS supported. Included in that is the
ability to `:extend`, and not only that, but `:extend(.mixin-name
all)`. By doing so, we remove duplicate CSS for all our elements that
were being clearfix-ed.

Fixes #8947, #8968, #8991, #9257, #9268, #9291, #9430, #9604, #9686,
#9929, #10731, #10793, #11305, #11498, #11533, #11570, #11604, #11652.

(dem issues, tho)

less/mixins.less

@@ -597,14 +597,14 @@
   margin-left: auto;
   padding-left:  (@grid-gutter-width / 2);
   padding-right: (@grid-gutter-width / 2);
-  .clearfix();
+  &:extend(.clearfix all);
 }
 
 // Creates a wrapper for a series of columns
 .make-row(@gutter: @grid-gutter-width) {
   margin-left:  (@gutter / -2);
   margin-right: (@gutter / -2);
-  .clearfix();
+  &:extend(.clearfix all);
 }
 
 // Generate the extra small columns