Second part of stock-integration.

Introduced StockArticle and a special page for ordering from stock. StockChanges will be created and the StockArticle.quantity
updated in 'order.close!'.

test/unit/stock_article_test.rb

@@ -0,0 +1,8 @@
+require 'test_helper'
+
+class StockArticleTest < ActiveSupport::TestCase
+  # Replace this with your real tests.
+  test "the truth" do
+    assert true
+  end
+end