Purpose of this commit is to remove the "done" field computation. Indeed
it is based on either
* mail_sent field if scheduler is global to the event (before or after
event). This computation is light as this field changes only once
when emails are scheduled and sent;
* status of event registrations compared to all sent communication on
those registrations. This is costly as adding a new registration changes
In this commit we therefore
* rename ``done`` to ``mail_done`` to ease grep and understanding;
* remove ``mail_sent`` as it is integrated within ``done``;
* manually update ``mail_done`` when updating schedulers instead of doing
it through a compute method;
Some code cleaning is performed to make it clearer and easier to understand.
LINKS
Task ID-2414658
COM PR odoo/odoo#63093
UPG PR odoo/upgrade#2014
Thibault Delavallée