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/views/orders/new.html.haml

@@ -1,10 +1,10 @@
 - title t('.title')
 
 = render 'form'
-
-- content_for :javascript do
-  :javascript
-    // select all articles by default
-    $(function() {
-      $('#checkall').click();
-    });
+- if @order.article_ids.empty?
+  - content_for :javascript do
+    :javascript
+      // select all articles by default
+      $(function() {
+        $('#checkall').click();
+      });