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/navbar.less

@@ -15,7 +15,7 @@
   border: 1px solid transparent;
 
   // Prevent floats from breaking the navbar
-  .clearfix();
+  &:extend(.clearfix all);
 
   @media (min-width: @grid-float-breakpoint) {
     border-radius: @navbar-border-radius;
@@ -29,7 +29,7 @@
 // styling of responsive aspects.
 
 .navbar-header {
-  .clearfix();
+  &:extend(.clearfix all);
 
   @media (min-width: @grid-float-breakpoint) {
     float: left;
@@ -54,7 +54,7 @@
   padding-left:  @navbar-padding-horizontal;
   border-top: 1px solid transparent;
   box-shadow: inset 0 1px 0 rgba(255,255,255,.1);
-  .clearfix();
+  &:extend(.clearfix all);
   -webkit-overflow-scrolling: touch;
 
   &.in {