[CRM] Migrate to new API

- Task: https://www.odoo.com/web#id=10543&view_type=form&model=project.task&action=333&active_id=248
- Pad: https://pad.odoo.com/p/r.qE00VdREVpboKWJp
again, `@api.model` (because `@api.multi` would try to pass `cr, uid, ids, context` as parameters and you do not need `ids` here)