feat(admin): add permission-based widget visibility on dashboard

- add optional `permission` field to `registerWidget` api
- filter widgets in `DashboardPage` based on user permissions
- register users widget with `users.view` permission requirement
- document `permission` parameter in admin README

src/features/admin/widgets/users.client.js

@@ -20,4 +20,4 @@ function UsersWidget({ data, loading }) {
   );
 }
 
-registerWidget({ id: 'users', Component: UsersWidget, order: 10 });
+registerWidget({ id: 'users', Component: UsersWidget, order: 10, permission: 'users.view' });