Update Font Awesome

Update Font Awesome to 4.6.1

stylesheets/font-awesome/less/mixins.less

@@ -12,15 +12,49 @@
 }
 
 .fa-icon-rotate(@degrees, @rotation) {
-  filter: progid:DXImageTransform.Microsoft.BasicImage(rotation=@rotation);
+  -ms-filter: "progid:DXImageTransform.Microsoft.BasicImage(rotation=@{rotation})";
   -webkit-transform: rotate(@degrees);
       -ms-transform: rotate(@degrees);
           transform: rotate(@degrees);
 }
 
 .fa-icon-flip(@horiz, @vert, @rotation) {
-  filter: progid:DXImageTransform.Microsoft.BasicImage(rotation=@rotation, mirror=1);
+  -ms-filter: "progid:DXImageTransform.Microsoft.BasicImage(rotation=@{rotation}, mirror=1)";
   -webkit-transform: scale(@horiz, @vert);
       -ms-transform: scale(@horiz, @vert);
           transform: scale(@horiz, @vert);
 }
+
+
+// Only display content to screen readers. A la Bootstrap 4.
+//
+// See: http://a11yproject.com/posts/how-to-hide-content/
+
+.sr-only() {
+  position: absolute;
+  width: 1px;
+  height: 1px;
+  padding: 0;
+  margin: -1px;
+  overflow: hidden;
+  clip: rect(0,0,0,0);
+  border: 0;
+}
+
+// Use in conjunction with .sr-only to only display content when it's focused.
+//
+// Useful for "Skip to main content" links; see http://www.w3.org/TR/2013/NOTE-WCAG20-TECHS-20130905/G1
+//
+// Credit: HTML5 Boilerplate
+
+.sr-only-focusable() {
+  &:active,
+  &:focus {
+    position: static;
+    width: auto;
+    height: auto;
+    margin: 0;
+    overflow: visible;
+    clip: auto;
+  }
+}