[IMP] mass_mailing: avoid to activate generic merge on mailing.list model

As the mailing.list model has his own custom merge method, this model should
not be candidate to activate the generic merge method comming from data_merge
module.

This change is done here to avoid creating a bridge module in enterprise
only to add this flag. We can consider that the flag is linked to the custom
merge method itself. If a model has a custom merge method, it should be marked
as 'having a custom merge method'.

Task ID: 2459416
Linked ENT PR: odoo/enterprise#17553

addons/mail/data/mail_templates.xml

@@ -351,8 +351,7 @@
         <a href="/web#action=mail.action_discuss&amp;active_id=mail.box_moderation" style="background-color: #1abc9c; padding: 20px; text-decoration: none; color: #fff; border-radius: 5px; font-size: 16px;" class="o_default_snippet_text">Moderate Messages</a>
         <br/><br/><br/>
     </div>
-    <p>Thank you!</p>
-</div>
+    <p>Thank you!</p</div>
         </template>
 
        <template id="mail_channel_send_guidelines">

addons/mass_mailing/models/mailing_list.py

@@ -11,6 +11,8 @@ class MassMailingList(models.Model):
     _order = 'name'
     _description = 'Mailing List'
     _mailing_enabled = True
+    # As this model has his own data merge, avoid to enable the generic data_merge on that model.
+    _disable_data_merge = True
 
     name = fields.Char(string='Mailing List', required=True)
     active = fields.Boolean(default=True)