[IMP] mail: allow to add cc emails when posting message

[JuMiSanAr]

In mail.thread, there's a _notify_email_recipient_values to allow to add additional values when posting a message.

However, to add an email_cc value, we also need to use it in the main method _notify_record_by_email.

[pedrobaeza]

This change is not valid, as this repo is only for patches. This will create a different behavior and features on OCB against vanilla Odoo.

Do your changes in a extension module.