[FIX] purchase: unlock PO
[FIX] purchase: unlock PO

Only allows a manager to unlock a PO. Otherwise, when the option
'Confirmed purchase orders are not editable', any user can unlock a PO
and modify it.

opw-707035

ygol/odoo,dfang/odoo,ygol/odoo,hip-odoo/odoo,ygol/odoo,hip-odoo/odoo,ygol/odoo,hip-odoo/odoo,dfang/odoo,dfang/odoo,dfang/odoo,dfang/odoo,ygol/odoo,ygol/odoo,dfang/odoo,hip-odoo/odoo,hip-odoo/odoo,ygol/odoo,hip-odoo/odoo