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/button-groups.less

@@ -37,7 +37,7 @@
 
 // Optional: Group multiple button groups together for a toolbar
 .btn-toolbar {
-  .clearfix();
+  &:extend(.clearfix all);
 
   .btn-group {
     float: left;
@@ -157,7 +157,7 @@
 
   // Clear floats so dropdown menus can be properly placed
   > .btn-group {
-    .clearfix();
+    &:extend(.clearfix all);
     > .btn {
       float: none;
     }

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

less/modals.less

@@ -98,7 +98,7 @@
   padding: (@modal-inner-padding - 1) @modal-inner-padding @modal-inner-padding;
   text-align: right; // right align buttons
   border-top: 1px solid @modal-footer-border-color;
-  .clearfix(); // clear it in case folks use .pull-* classes on buttons
+  &:extend(.clearfix all); // clear it in case folks use .pull-* classes on buttons
 
   // Properly space out buttons
   .btn + .btn {

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 {

less/navs.less

@@ -10,7 +10,7 @@
   margin-bottom: 0;
   padding-left: 0; // Override default ul/ol
   list-style: none;
-  .clearfix();
+  &:extend(.clearfix all);
 
   > li {
     position: relative;

less/pager.less

@@ -8,7 +8,7 @@
   margin: @line-height-computed 0;
   list-style: none;
   text-align: center;
-  .clearfix();
+  &:extend(.clearfix all);
   li {
     display: inline;
     > a,

less/panels.less

@@ -15,7 +15,7 @@
 // Panel contents
 .panel-body {
   padding: 15px;
-  .clearfix();
+  &:extend(.clearfix all);
 }
 
 

less/type.less

@@ -202,7 +202,7 @@ dd {
     }
     dd {
       margin-left: @component-offset-horizontal;
-      .clearfix(); // Clear the floated `dt` if an empty `dd` is present
+      &:extend(.clearfix all); // Clear the floated `dt` if an empty `dd` is present
     }
   }
 }