[FIX] mail: avoid attaching auto-notifications to foreign parents

`parent_id` fields are common in many models, and thus default values
for those fields are sometimes passed in the context.

Because mail.message also has `parent_id` field, it would automatically
use the default when an automatic message was being posted. While of
course, the parent_id value comes from a different model.
This "adoption" by a random "parent message" is unexpected,
not desired, and it can even cause a very surprising AccessError if the
parent message is not readable by the user.

Forcing the `parent_id` value during the creation of an automatic message
avoids this confusion.

One way to trigger the bug was to use the "subtask" stat button to create a
child subtask for a project task (it relies on the parent task ID
passed in the context)

addons/mail/models/mail_thread.py

@@ -2067,6 +2067,7 @@ class MailThread(models.AbstractModel):
                 partner_ids=[(4, pid) for pid in partner_ids],
                 auto_delete=True,
                 auto_delete_message=True,
+                parent_id=False, # override accidental context defaults
                 subtype_id=self.env.ref('mail.mt_note').id)
 
     @api.multi