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/components/PermissionGrid.js

@@ -44,7 +44,7 @@ export default class PermissionGrid extends Component {
             </tr>
             {section.children.map(child => (
               <tr className="PermissionGrid-child">
-                <th>{child.icon ? icon(child.icon) : ''}{child.label}</th>
+                <th>{icon(child.icon)}{child.label}</th>
                 {permissionCells(child)}
                 <td/>
               </tr>