[FIX] event_sale: fix event registration confirmation modal
[FIX] event_sale: fix event registration confirmation modal

Task #1910927

Purpose
=======

When confirming a sale order with event tickets, a modal is supposed to be shown to be able to
confirm the event registration.

That modal was not shown anymore.

To solve it, we split the '_action_confirm' to run the actual business of the event_sale module,
which is '_update_registrations', and moved the action returned in the regular 'action_confirm' method.

closes odoo/odoo#29997
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo