perf(solidus_admin): Reduce N+1 queries for order index

There was a N+1 query in order index view. For each row the quantity of the line items was fetched. It is now included and reduces the amount of SQL queries.
This change reduced the loading time of 8M orders from 836ms to 520ms.

Author: sascha-karnatz

admin/app/components/solidus_admin/orders/index/component.rb

@@ -162,7 +162,7 @@ def items_column
     {
       header: :items,
       data: ->(order) do
-        content_tag :div, t(".columns.items", count: order.line_items.sum(:quantity))
+        content_tag :div, t(".columns.items", count: order.line_items.sum(&:quantity))
       end
     }
   end

admin/app/controllers/solidus_admin/orders_controller.rb

@@ -13,7 +13,7 @@ class OrdersController < SolidusAdmin::BaseController
 
     def index
       orders = apply_search_to(
-        Spree::Order,
+        Spree::Order.includes(:line_items),
         param: :q,
         distinct: false
       )