purchase_tier_validation can't receive quantity manually

[michaelslade54]

Module

purchase_tier_validation (although suspect it is actually change in base_tier_validation)

Describe the bug

With purchase_tier_validation turned on and a validation requirement in place, once the Purchase Order is validated and confirmed, the qty_received cannot be updated for things like service items, as the order_lines are read-only. The tier_validation_exception mechanism does not work, as you can only select the entire order_line (and therefore can change prices etc).

To Reproduce

Affected versions: v16

Steps to reproduce the behavior:

1. Create a basic tier validation definition for all goods over $0 and assign the admin as the validating user.
2. Create a service product that has invoice policy set to "Delivered Quantities" and cost $10.
3. Add service product to a purchase order, validate and confirm the order.
4. Try to set the quantity received to the ordered quantity (so that bills can be created against it). As the entire order_line table is read-only, the service cannot be marks as received.

Expected behavior
The qty received should not be a read-only field and should be freely editable after validation is complete.

Additional context
I believe this change OCA/server-ux#860 has broken this previously working functionality by making all ancestor fields read only, with no way to override any fields contain in a linked model (i.e. purchase.order.line)