Improve concern loading

It shouldn't need `require` or `prepend`

app/models/concerns/order_shipment.rb

@@ -13,7 +13,9 @@ require 'active_support/concern'
 module OrderShipment
   extend ActiveSupport::Concern
 
-  attr_accessor :manual_shipping_selection
+  included do
+    attr_accessor :manual_shipping_selection
+  end
 
   def shipment
     shipments.first

app/models/spree/order.rb

@@ -4,12 +4,10 @@ require 'spree/order/checkout'
 require 'open_food_network/enterprise_fee_calculator'
 require 'open_food_network/feature_toggle'
 require 'open_food_network/tag_rule_applicator'
-require 'concerns/order_shipment'
 
 module Spree
   class Order < ApplicationRecord
-    prepend OrderShipment
-
+    include OrderShipment
     include Checkout
     include Balance