FIX: Plugin JS failing to load would break admin interface (#29139)

If a plugin's JS fails to load for some reason, most commonly
ad blockers, the entire admin interface would break. This is because
we are adding links to the admin routes for plugins that define
them in the sidebar.

We have a fix for this already in the plugin list which shows a warning
to the admin. This fix just prevents the broken link from rendering
in the sidebar if the route is not valid.

lib/plugin/instance.rb

@@ -117,6 +117,18 @@ class Plugin::Instance
     }
   end
 
+  def full_admin_route
+    route = self.admin_route
+    return unless route
+
+    route
+      .slice(:location, :label, :use_new_show_route)
+      .tap do |admin_route|
+        path = admin_route[:use_new_show_route] ? "show" : admin_route[:location]
+        admin_route[:full_location] = "adminPlugins.#{path}"
+      end
+  end
+
   def configurable?
     true
   end