Using this mixin allows to hide details about jinja-based computation for
main fields used when posting messages or sending emails. Notably subject
and body computation that was duplicated in several addons are now done
in a mixin. It also eases their maintenance or evolution when qweb rendering
will be available. It also eases support of lang when necessary.
``mail.compose.message`` is still overriding default mixin behavior to keep
a minimal fix. Rewriting composer code is planned but will not be done with
this task. Here we keep current behavior, technically and functionally.
Task ID-2534550 (Template usage improvement)
Task ID-2477164 (Composer mixin)
COM PR odoo#70889
ENT PR odoo/enterprise#18352
UPG PR odoo/upgrade#2496
Co-Authored-By: Stéphane Debauche <std@odoo.com>
Co-Authored-By: Thibault Delavallée <tde@odoo.com>
