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/registry.js

@@ -25,8 +25,8 @@ export function registerWidgetFetcher(id, fetcher) {
   widgetFetchers.set(id, fetcher);
 }
 
-export function registerWidget({ id, Component, order = 0 }) {
-  widgetComponents.set(id, { Component, order });
+export function registerWidget({ id, Component, order = 0, permission }) {
+  widgetComponents.set(id, { Component, order, permission });
 }
 
 export function getWidgets() {