Only disable pointer-events on disabled <a> btns; fixes #16088

<a> doesn't support the `[disabled]` attribute,
so `a.btn.disabled` simulates it using `pointer-events: none`.
However, this is unnecessary for <button>s and <input>s, and
also prevents their `[disabled]` cursor from displaying.

[skip sauce]
[skip validator]

less/buttons.less

@@ -47,10 +47,16 @@
   &[disabled],
   fieldset[disabled] & {
     cursor: @cursor-disabled;
-    pointer-events: none; // Future-proof disabling of clicks
     .opacity(.65);
     .box-shadow(none);
   }
+
+  a& {
+    &.disabled,
+    fieldset[disabled] & {
+      pointer-events: none; // Future-proof disabling of clicks on `<a>` elements
+    }
+  }
 }