[FIX] mass_mailing: markup usage

- using a string to join `Markup`s is useless, that just strips out
  the `Markup`
- the entire toplevel `Markup` can be formatted in one shot

closes odoo#140428

Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>

addons/mass_mailing/models/mailing_list.py

@@ -340,17 +340,15 @@ def _update_subscription_from_email(self, email, opt_out=True, force_message=Non
             if force_message:
                 body = force_message
             elif opt_out:
-                body = Markup('<p>%s</p>') % _(
-                    '%(contact_name)s unsubscribed from the following mailing list(s)',
-                    contact_name=contact.display_name
+                body = Markup('<p>%s</p><ul>%s</ul>') % (
+                    _('%(contact_name)s unsubscribed from the following mailing list(s)', contact_name=contact.display_name),
+                    Markup().join(Markup('<li>%s</li>') % name for name in updated.mapped('name')),
                 )
-                body += Markup('<ul>%s</ul>' % ''.join(Markup('<li>%s</li>') % name for name in updated.mapped('name')))
             else:
-                body = Markup('<p>%s</p>') % _(
-                    '%(contact_name)s subscribed to the following mailing list(s)',
-                    contact_name=contact.display_name
+                body = Markup('<p>%s</p><ul>%s</ul>') % (
+                    _('%(contact_name)s subscribed to the following mailing list(s)', contact_name=contact.display_name),
+                    Markup().join(Markup('<li>%s</li>') % name for name in updated.mapped('name')),
                 )
-                body += Markup('<ul>%s</ul>' %  ''.join(Markup('<li>%s</li>') % name for name in updated.mapped('name')))
             contact.with_context(mail_create_nosubscribe=True).message_post(
                 body=body,
                 subtype_id=self.env['ir.model.data']._xmlid_to_res_id('mail.mt_note'),