[IMP] mail: add a post-process hook for message post

Models inheriting from mail.thread can override this method and hook
on post message_post event and perform some action depending on the
business flow.

Future commits will use that hook to update the record after posting
a message, like updating the customer.

addons/mail/models/mail_thread.py

@@ -1894,8 +1894,14 @@ class MailThread(models.AbstractModel):
                 self.sudo().write({'message_last_post': fields.Datetime.now()})
         if new_message.author_id and model and self.ids and message_type != 'notification' and not self._context.get('mail_create_nosubscribe'):
             self.message_subscribe([new_message.author_id.id], force=False)
+
+        self._message_post_after_hook(new_message)
+
         return new_message
 
+    def _message_post_after_hook(self, message):
+        pass
+
     @api.multi
     def message_post_with_view(self, views_or_xmlid, **kwargs):
         """ Helper method to send a mail / post a message using a view_id to