Fix(Promotions): Instantiate single promo code with promotion

Because promotion codes have the `-> {with_discarded}` scope on the
`promotion` association, a promo code that's instantiated with
`promotion.codes.new` will not have the promotion associated at the
beginning, leading to errors. This passes in the promotion so that the
promo code knows where it belongs.

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

@@ -11,8 +11,7 @@ class PromotionsController < BaseController
 
       def create
         @promotion = model_class.new(permitted_resource_params)
-        @promotion.codes.new(value: params[:single_code]) if params[:single_code].present?
-
+        @promotion.codes.new(promotion: @promotion, value: params[:single_code]) if params[:single_code].present?
         if params[:code_batch]
           @code_batch = @promotion.code_batches.new(code_batch_params)
         end