[REF] EventMaster event split tickets psa

 - Task: https://www.odoo.com/web#id=8707&view_type=form&model=project.task&action=333&active_id=131
 - Pad: https://pad.odoo.com/p/r.cUcnzS776UUvQn84

Rationale
- when customer buys X tickets for an event
-- sale order: 1 line, with correct quantity
-- registrations: X lines
- purpose
-- when buying tickets: screen to add details about registrations
-- currently: subscription with quantity -> quantity not relevant anymore
Why not `self.event_id.seats_max and self.event_id.seats_available < self.event_id.seats_max` ?