[FIX] mass_mailing: avoid creating mass mailing for messages posts

In case a default value is set for the field `mass_mailing_name` of the
mail composer, the automatic messages sent via message_post_with_* are
considered as a mass mailing. It may also create an AccessError if the
current user is not a mass mailing user.

In the case of the "assign user" email (for auto-subscribed users), the
url wasn't shorten, which resulted in invalid links (opw-752941) (fixed
by 3228694bdd)

addons/mass_mailing/models/mail_thread.py

@@ -39,3 +39,13 @@ class MailThread(models.AbstractModel):
             message_ids = [x.strip() for x in decode_smtp_header(message['References']).split()]
             self.env['mail.mail.statistics'].set_replied(mail_message_ids=message_ids)
         return super(MailThread, self).message_route_process(message, message_dict, routes)
+
+    @api.multi
+    def message_post_with_template(self, template_id, **kwargs):
+        # avoid having message send through `message_post*` methods being implicitly considered as
+        # mass-mailing
+        no_massmail = self.with_context(
+            default_mass_mailing_name=False,
+            default_mass_mailing_id=False,
+        )
+        return super(MailThread, no_massmail).message_post_with_template(template_id, **kwargs)