[IMP] mail: context key for message author subscription

By default the author of a message is always removed from the notified recipients.
However in some cases it is interesting to be able to notify the author. A context
key has been added that handle that case.

addons/mail/models/mail_message.py

@@ -836,7 +836,7 @@ class Message(models.Model):
             partners = self_sudo.partner_ids
 
         # remove author from notified partners
-        if self_sudo.author_id:
+        if not self._context.get('mail_notify_author', False) and self_sudo.author_id:
             partners = partners - self_sudo.author_id
 
         # update message