Do not subtract articles in open orders when calculating quantity.
Revert "Do not subtract articles in open orders when calculating quantity."

This reverts commit 6bf3b241e6a34b7ce5a5d704df5dc1573ad40951.
Do not exclude ordered articles (with zero quantity) when editing orders.
Merge branch 'rails3-fix-stock-ordering' into rails3-fix-stock-ordering-2