[FIX] website_sale: set confirmation mail template setting to readonly

In website_sale setting, there is a field 'Confirmation Email' that is not used,
and which purpose is to have a conveninent link to edit the mail templates.
Since it is an editable setting, this confuses the users.
It was in readonly before commit:
https://github.com/odoo/odoo/commit/65a7531c8b5c713969e49a5fab9d4672524c3ba8#diff-3f25fe0e27881e9c6655bbb6974c84abR59
which removed the readonly without any apparent reason.

We put it back into readonly mode.

opw 1860167

addons/website_sale/models/res_config_settings.py

@@ -43,7 +43,7 @@ class ResConfigSettings(models.TransientModel):
 
     order_mail_template = fields.Many2one('mail.template', string='Order Confirmation Email',
         default=_default_order_mail_template, domain="[('model', '=', 'sale.order')]",
-        help="Email sent to customer at the end of the checkout process")
+        help="Email sent to customer at the end of the checkout process", readonly=True)
 
     automatic_invoice = fields.Boolean("Automatic Invoice",
                                        help="The invoice is generated automatically and available in the customer portal "