This commits introduces a new mixin ``mail.composer.mixin`` used when sending emails or notifications based on a mail template. Main current purpose is to hide details related to subject and body computation and rendering based on a mail.template. It also give the base tools to control who is allowed to edit body, notably when dealing with templating language like jinja or qweb. It is meant to evolve in a near future with upcoming support of qweb and fine grain control of rendering access. Task ID-2534550 (Template usage improvement) Task ID-2477164 (Composer mixin) COM PR odoo/odoo#70889 ENT PR odoo/enterprise#18352 Co-Authored-By: Stéphane Debauche <std@odoo.com> Co-Authored-By: Thibault Delavallée <tde@odoo.com>