Keep member order on orderarticle deletion

When an article is deleted in the balancing screen, the associated GroupOrderArticles are also deleted, and thus information is lost. This operation cannot be undone.

We have found this to happen when an article was unavailable, and so it was deleted from an order. Then the supplier had a slightly different article as a substitution. We had to ask which member ordered the original article to redistribute.

This patch sets the amount ordered to zero when any GroupOrderArticle is associated.