Fix(promotions): Add model_class method to PromotionCodesController

Because the new promotion system lives outside the `spree` namespace, we
need to be diligent about adding `model_class` to all the controllers.

I have not added a spec, because this only fails if
solidus_legacy_promotions is not loaded, and that is not the case in our
test suite.

promotions/lib/controllers/backend/solidus_promotions/admin/promotion_codes_controller.rb

@@ -53,6 +53,10 @@ def load_promotion
           .accessible_by(current_ability, :show)
           .find(params[:promotion_id])
       end
+
+      def model_class
+        SolidusPromotions::PromotionCode
+      end
     end
   end
 end