CSRUI: Don't allow editing of shipment line item if shipment completed/canceled

Description

Steps to reproduce

1. Create a multi shipment order.
2. Cancel or complete one shipment.
3. In the CSR UI, select the order.
4. Select the cancelled/completed shipment.
5. Add a new line item or update the quantity of an existing item.

Expected:
Should not allow editing of a line item from a completed/cancelled shipment

Actual:
Can add and update a line item a completed/cancelled shipment