[accounting] New API migration

- Task: https://www.odoo.com/web/#id=9955&view_type=form&model=project.task&action=333&active_id=131
- Pad: https://pad.odoo.com/p/r.Uy4wEA7I4G4GwPaU
to be checked: if this is a constraint, use @api.constrains, if this is @api.multi, it should work on a collection of records