Import legacy items after routes are generated

When importing legacy menu items into the new storefront's API, the
`import_menu_items_from_backend` method will try reading the legacy menu
item's paths. Those path might be defined in lambdas referencing route
helpers, but those route helper are only available after the
application's routes have been fully loaded.

I understand this is somewhat kludgy, but on the upside: It works.

admin/lib/generators/solidus_admin/install/templates/config/initializers/solidus_admin.rb.tt

@@ -14,9 +14,15 @@ SolidusAdmin::Config.configure do |config|
   # you can import menu_items from the backend by uncommenting the following line,
   # but you will need to
   <%- if defined? Spree::Backend -%>
-  config.import_menu_items_from_backend!
+  Rails.application.config.after_initialize do
+    Rails.application.reload_routes!
+    config.import_menu_items_from_backend!
+  end
   <%- else -%>
-  # config.import_menu_items_from_backend!
+  # Rails.application.config.after_initialize do
+  #  Rails.application.reload_routes!
+  #  config.import_menu_items_from_backend!
+  # end
   <%- end -%>
 
   # Add custom paths to importmap files to be loaded.