[FIX] mail: notify 'inbox' partners

When
- a user A had set 'notifications handled by Odoo'
- a user B sent a @userA message

Before this commit, the user wasn't notified in the inbox needaction

After this commit, he is

OPW 801315

addons/mail/models/mail_message.py

@@ -845,6 +845,9 @@ class Message(models.Model):
             ('channel_ids', 'in', email_channels.ids),
             ('email', '!=', self_sudo.author_id and self_sudo.author_id.email or self_sudo.email_from),
         ])._notify(self, force_send=force_send, send_after_commit=send_after_commit, user_signature=user_signature)
+
+        notif_users.mapped('partner_id')._notify_by_chat(self)
+
         channels_sudo._notify(self)
 
         # Discard cache, because child / parent allow reading and therefore