[FIX] account,sale: restore record assignment notifications

All accross Odoo, when a document is assigned to a salesperson, that person is notified via email.

However, the `account.move` model is inconsistent with that behavior, because the salesperson field name is `invoice_user_id` instead of just `user_id`.

With this fix:

- When creating an invoice from a sales order, the salesperson will get the assignment notification.
- When modifying the salesperson in the invoice form, the new salesperson will get the assignment notification.
- In both cases, the salesperson will get subscribed to the invoice.

This restores the behavior that `account.invoice` had in v12. For master, a better fix should be considered (a.k.a. simply removing the `invoice_user_id` field and using only `user_id` to avoid these kind of inconsistencies).

@Tecnativa TT29909

closes odoo/odoo#71328

Signed-off-by: oco-odoo <oco-odoo@users.noreply.github.com>