Don't allow line items for a completed order to be created, updated or d...

...estroyed

This is an attempt to fix a problem discussed in IRC where a line item can be added or removed from a completed order via the api, causing the order to change from completed to an address state, and causing all shipments to be removed from the order.

This appears to create multiple other test failures, however, I will be offline for a few days so I figured I'd provide this to the community as an initial pass.