LP#1966096: check permissions when deleting lineitem

LP#1966096: check permissions when deleting lineitem

Hitherto EG has only prevented you from deleting a lineitem if it's
attached to a picklist belonging to someone else (in which case the
deletion would silently fail).  This commit checks whether you have the
CREATE_PURCHASE_ORDER perm if the line item is attached to a purchase
order, or else the CREATE_PICKLIST perm if the line item is attached to
a picklist.

Signed-off-by: Jeff Davis <jeff.davis@bc.libraries.coop>
Signed-off-by: Eva Cerniňáková <cerninakova@gmail.com>
Signed-off-by: Jane Sandberg <js7389@princeton.edu>