Adds a {{disabled}} modifier that works similar to fieldset

In that you can set it to false on a parent node and all its sub nodes
will be disabled. It has an added improvement that you can then separate
allow children if you want to disable everything apart from one thing.

One caveat here is that it relies on dom twiddling and is therefore
disabling a parent is not currently very reactive and will need some
MutationObserver work to make it fully reactive.

ui/packages/consul-ui/app/modifiers/disabled.js

@@ -0,0 +1,17 @@
+import { modifier } from 'ember-modifier';
+
+export default modifier(function enabled($element, [bool], hash) {
+  if (['input', 'textarea', 'select', 'button'].includes($element.nodeName.toLowerCase())) {
+    if (bool) {
+      $element.disabled = bool;
+    } else {
+      $element.dataset.disabled = false;
+    }
+    return;
+  }
+  for (const $el of $element.querySelectorAll('input,textarea')) {
+    if ($el.dataset.disabled !== 'false') {
+      $el.disabled = bool;
+    }
+  }
+});