[IMP] mail: add a hook in _notify_record_by_email to add model-specific values

PURPOSE

Be able to customize notification email values in a given model by overriding
a simple method instead of having to hijack the email creation process.

SPECIFICATIONS

Add a hook in mail.thread that can be overridden in specific addons, either
by model or globally by inheriting mail.thread.

Include _notify_email_headers in it so that it is correctly embedded in
model-specific values computation.

This removes the need to replace _notify_record_by_email long method in a
custom module.

Closes odoo/odoo#48213
Closes odoo/odoo#50097

closes odoo/odoo#50129

X-original-commit: 1ba8712d7a797cf2e821ae881cb1f1c4ea2fc11d
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/mail/models/mail_thread.py

@@ -2270,9 +2270,7 @@ class MailThread(models.AbstractModel):
             'references': message.parent_id.sudo().message_id if message.parent_id else False,
             'subject': mail_subject,
         }
-        headers = self._notify_email_headers()
-        if headers:
-            base_mail_values['headers'] = headers
+        base_mail_values = self._notify_by_email_add_values(base_mail_values)
 
         Mail = self.env['mail.mail'].sudo()
         emails = self.env['mail.mail'].sudo()
@@ -2425,6 +2423,19 @@ class MailThread(models.AbstractModel):
             'lang': lang,
         }
 
+    def _notify_by_email_add_values(self, base_mail_values):
+        """ Add model-specific values to the dictionary used to create the
+        notification email. Its base behavior is to compute model-specific
+        headers.
+
+        :param dict base_mail_values: base mail.mail values, holding message
+        to notify (mail_message_id and its fields), server, references, subject.
+        """
+        headers = self._notify_email_headers()
+        if headers:
+            base_mail_values['headers'] = headers
+        return base_mail_values
+
     def _notify_compute_recipients(self, message, msg_vals):
         """ Compute recipients to notify based on subtype and followers. This
         method returns data structured as expected for ``_notify_recipients``. """