Make mail_notification_layout as the default mail template and remove
message_notification_email. We now only have 2 main notification mail layout
templates (outside of a light one): the default one and one that inherits from
it to only change the signature to the responsible instead of the author one.
In this operation the layout of message_notification_email has been integrated
in mail_notification_layout to support outlook mail client for which
message_notification_email has just been reworked (odoo/odoo#92847).
Technical note: we have integrated the "internal communication" banner from
"message_notification_email" into the new default template
"mail_notification_layout" which cause one added query in the test for querying
the flag internal from mail.message.subtype.
Task-2801600
Part-of: odoo/odoo#88466
Pierre-Yves Dufays
bee4a2659d