Add possibility to copy an order

If a supplier has a long list of articles, which are all available, but
the order should contain only a subset of them, selecting them for every
order can take a long time. Starting with a copy of an existing order
can safe a lot of time.

app/controllers/orders_controller.rb

@@ -63,6 +63,7 @@ class OrdersController < ApplicationController
   # Page to create a new order.
   def new
     @order = Order.new(supplier_id: params[:supplier_id]).init_dates
+    @order.article_ids = Order.find(params[:order_id]).article_ids if params[:order_id]
   end
 
   # Save a new order.