[IMP] mail, auth_signup: correctly categorize emails

Mail.message holds a 'message_type' selection field allowing to categorize
the usage of the message: email (incoming emails), comment (user generated
input), notification (system generated input) and user_notification.

This one indicates the message (or mail) targets some specific recipients
and has notably specific display tweaks, to avoid bloating the chatter.

In this commit we fix category of some emails, setting as notifications
when it has no generic usage on a given document.

Task-2710804 (Mail: Clean MailThread API)

closes odoo/odoo#111781

X-original-commit: 82374eefd536a1090be5a0845956a0d7cf493cb0
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/auth_signup/models/res_users.py

@@ -187,6 +187,7 @@ class ResUsers(models.Model):
         email_values = {
             'email_cc': False,
             'auto_delete': True,
+            'message_type': 'user_notification',
             'recipient_ids': [],
             'partner_ids': [],
             'scheduled_date': False,