manager: use administrate helper to retrieve resource path

This is lifted from the current default template provided by
administrate.

app/views/manager/application/_navigation.html.erb

@@ -16,7 +16,7 @@ as defined by the routes in the `admin/` namespace
 
     <%= link_to(
       display_resource_name(resource),
-      [namespace, resource.path],
+      resource_index_route(resource),
       class: "navigation__link navigation__link--#{nav_link_state(resource)}"
     ) %>
   <% end %>