Handle order items and valuation items when deleting budget items and budget groups. #220
Description
When a budgetitem/budgetgroup that has an orderitem/valuationitem is deleted, editing the order/valuation results in an error and it can't be edited. I think it should be handled differently than in #184. A budgetitem that has an orderitem should be able to be deleted, and the orderitem should keep its state separate from the budgetitem.