RATIONALE
Naming conventions
* content = core message / subject (+ other fields);
* email layout = layouting applied to messages when sent to recipients who
receive notifications by email e.g. access button, ...;
* comment mode / mailing mode: composer main composition mode: posting on
record(s) / sending a mailing on records;
Content situation when using a template to post on a composer
* comment, monorecord mode: choose a template, content is rendered directly
from template side according to 'lang' field definition -> ok;
* comment, multirecord mode / mailing mode: choose a template, content is
the raw content. At post / send, content is rendered and translated if
matching template content -> ok;
Email layout situation when posting / mailing
* comment: layout translated based on recipient lang or fallback on template
'lang' field definition, or current user's lang;
* mailing: no layout supported;
PURPOSE
Make those the two composer modes less different by supporting layouting in
mailing mode of composer. Consider it a bit experimental.
SPECIFICATIONS
When using the composer in mailing mode, render email layout if given. This
layout is used to encapsulate body.
To simplify rendering and sending, consider all recipients to be 'customer'
as defined in '_notify_get_recipients'. This means all recipients receive the
same basic layouting currently as a first attempt to improve this situation.
Considered lang is the one coming from the template 'lang' field, or fallback
on current user's lang.
Task-3186426 (Mail: Support notification layout in template / email composer)
Part-of: odoo/odoo#106177
Thibault Delavallée
c2f8b03e3f