Master website event sale seats availability psa tde

- Task: https://www.odoo.com/web?#id=9971&view_type=form&model=project.task&menu_id=3942&action=327
- Pad: https://pad.odoo.com/p/r.BTJ9yFYnbxBG3VzQ
As far as I could see, this is only called from a new-API `create` which returns a singleton recordset, so

* `@api.multi` is probably unnecessary since this is only called from a new API context
* the iteration could be removed (leading to the method erroring out if the method is ever called on a non-singleton recordset)