Fix UAs required message position on grouped radio buttons

Instead of not rendering the element at all with display:none, use opacity:0 and z-index:-1 so the radio element has a defined position on the document and user agents can show the required message in the right place.

less/button-groups.less

@@ -222,5 +222,7 @@
 // Checkbox and radio options
 [data-toggle="buttons"] > .btn > input[type="radio"],
 [data-toggle="buttons"] > .btn > input[type="checkbox"] {
-  display: none;
+  opacity: 0;
+  position: absolute;
+  z-index: -1;
 }