ACQ lineitem history view fails on deleted picklist

Evergreen 2.5+

Steps to reproduce:

1. Create a picklist with a lineitem
2. Add the lineitem to a PO
3. Delete the original picklist
4. In the PO view, choose "View History" in the lineitem actions selector.
5. The history page fails to display any entries.

Reported error:

Status: open-ils.pcrud: no object of class acqpl found with primary key id of 6

Method:

open-ils.pcrud open-ils.pcrud.search.acqlih "AUTH", {"id":"16"}, {"order_by":{"acqlih":"audit_time DESC"},"limit":15}

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It's valid for a historical lineitem entry to point to a now-deleted picklist, but PCRUD retrieval fails, since it attempts to perform a permission check (context org unit lookup) using the deleted picklist. The history retrieval code should probably be moved into a proper API call that performs a single permission check on the current form of the lineitem and returns all history objects, regardless of their previous state.