Improve permissions page

- Introduce the concept of "required permissions" - basically a permission dependency tree. In order for a group to be granted one permission, they must also have another.
- Improve redraw performance by not building dropdown menu contents until dropdown is opened

ref #904

js/admin/src/app.js

@@ -14,4 +14,20 @@ app.initializers.add('boot', boot, -100);
 
 app.extensionSettings = {};
 
+app.getRequiredPermissions = function(permission) {
+  const required = [];
+
+  if (permission === 'startDiscussion' || permission.indexOf('discussion.') === 0) {
+    required.push('viewDiscussions');
+  }
+  if (permission === 'discussion.delete') {
+    required.push('discussion.hide');
+  }
+  if (permission === 'discussion.deletePosts') {
+    required.push('discussion.editPosts');
+  }
+
+  return required;
+};
+
 export default app;