remember member order when order article is deleted

When an article is deleted in the balancing screen, the associated `GroupOrderArticle`s 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.

@bennibu / @fsmanuel could you please check if this does not break anything I didn't think of?