[project] Onboarding

- Task: https://www.odoo.com/web/#id=10467&view_type=form&model=project.task&action=333&active_id=131
- Pad: https://pad.odoo.com/p/r.5sIeAQv7J7qXYCmI
@tde-banana-odoo  because, when a task is created from SO : date_deadline should be blank in task form as per SPEC.