Add #authorization_subject to Promotions controllers

The  `authorization_subject` method from `solidus_admin` assumes that
all models are in the `Spree` namespace.

(cherry picked from commit c2ba46a)

promotions/lib/controllers/admin/solidus_promotions/promotion_categories_controller.rb

@@ -25,5 +25,11 @@ def destroy
       flash[:notice] = t(".success")
       redirect_back_or_to solidus_promotions.promotion_categories_path, status: :see_other
     end
+
+    private
+
+    def authorization_subject
+      SolidusPromotions::PromotionCategory
+    end
   end
 end

promotions/lib/controllers/admin/solidus_promotions/promotions_controller.rb

@@ -42,5 +42,9 @@ def load_promotion
     def promotion_params
       params.require(:promotion).permit(:user_id, permitted_promotion_attributes)
     end
+
+    def authorization_subject
+      SolidusPromotions::Promotion
+    end
   end
 end