RATIONALE
Currently we can specify email used for notification layouting through context
use in mail composer. It is then propagated to message_post, stored on
mail.message and used to encapsulate emails sent based on posted messages.
SPECIFICATIONS
Get rid of context usage (``custom_layout``) and use a real field on composer
model: ``email_layout_xmlid``. Use now a default value coming from context
(default_email_layout_xmlid) instead of custom_layout.
Support old context key in composer for backward compatibility, working like
a default value for the field itself.
Task-2621326 (Mail: add 'view' button in 'light notification template')
Task-2647302 (Mail: add layout field in composer)
UPG odoo/upgrade#2829
Part-of: odoo/odoo#76418
Thibault Delavallée
3659546738