Add stock group order

This allows us to add additional items to an order, which do not belong
a specific user, but will be put into stock. The benefit of this change
is that we use the same order for ordergroups and stock.

db/migrate/20160224201529_allow_stock_group_order.rb

@@ -0,0 +1,9 @@
+class AllowStockGroupOrder < ActiveRecord::Migration
+  def self.up
+    change_column :group_orders, :ordergroup_id, :integer, :default => nil, :null => true
+  end
+
+  def self.down
+    change_column :group_orders, :ordergroup_id, :integer, :default => 0, :null => false
+  end
+end