Purpose is to ease understanding of those default values and prepare move
towards editable stored computed fields by rewriting a bit the code to better
understand its purpose.
While being at it, some tests introduced recently are moved at their right
place now that everything is ready to rewrite the posting API and the
composer. Some tests are added to ensure notification-specific methods are
called along with the newly-introduced '_message_compute_subject', aka methods
to add custom header and custom mail values.
Task-2710804 (Mail: Clean MailThread Posting API)
Prepares Task-2088884 (Mail: Use editable computed stored fields in compo
Part-of: odoo/odoo#99482
Thibault Delavallée
041e9924c7