add block level input mixin for border-box box-sizing

less/responsive.less

@@ -182,14 +182,7 @@
   select[class*="span"],
   textarea[class*="span"],
   .uneditable-input {
-    display: block;
-    width: 100%;
-    min-height: 28px; /* Make inputs at least the height of their button counterpart */
-    /* Makes inputs behave like true block-level elements */
-    -webkit-box-sizing: border-box; /* Older Webkit */
-       -moz-box-sizing: border-box; /* Older FF */
-        -ms-box-sizing: border-box; /* IE8 */
-            box-sizing: border-box; /* CSS3 spec*/
+    .input-block-level();
   }
   // But don't let it screw up prepend/append inputs
   .input-prepend input[class*="span"],