[A11Y] Make checkboxes focusable (#3014)

* Add extra feature to a11y focusring mixin

* Add visually hidden CSS class and mixin

* Visually hide checkboxes (keep in focus/a11y tree)

* Place checkbox focus ring around display element

* Improve mobile checkbox/switch accessibility

less/common/scaffolding.less

@@ -148,3 +148,13 @@ blockquote ol:last-child {
   font-size: 18px;
   color: @muted-more-color;
 }
+
+.visually-hidden {
+  clip: rect(0 0 0 0);
+  clip-path: inset(50%);
+  height: 1px;
+  overflow: hidden;
+  position: absolute;
+  white-space: nowrap;
+  width: 1px;
+}