UX: Change admin plugins list to follow UI guidelines (#28478)

This commit introduces a little bit of duplication
since the old plugin UIs not using the new plugin show
page look different from ones like AI and Gamification
which have been converted. We can use the new admin
header component on the plugins list, but for the other
pages we are manually rendering a breadcrumb trail and
the list of plugin tabs.

Over time as we convert more plugins to use the new UI
guidelines and show page we can get rid of this duplication.

app/assets/javascripts/admin/addon/controllers/admin-plugins.js

@@ -13,19 +13,21 @@ export default class AdminPluginsController extends Controller {
     return this.allAdminRoutes.filter((route) => !this.routeExists(route));
   }
 
+  // NOTE: See also AdminPluginsIndexController, there is some duplication here
+  // while we convert plugins to use_new_show_route
   get allAdminRoutes() {
     return this.model
-      .filter((plugin) => plugin?.enabled)
+      .filter((plugin) => plugin?.enabled && plugin?.adminRoute)
       .map((plugin) => {
-        return plugin.adminRoute;
-      })
-      .filter(Boolean);
+        return Object.assign(plugin.adminRoute, { plugin_id: plugin.id });
+      });
   }
 
   get showTopNav() {
     return (
-      !this.adminPluginNavManager.currentPlugin ||
-      this.adminPluginNavManager.isSidebarMode
+      !this.adminPluginNavManager.viewingPluginsList &&
+      (!this.adminPluginNavManager.currentPlugin ||
+        this.adminPluginNavManager.isSidebarMode)
     );
   }