When an online appointment (having no location set in it's appointment type) is booked, we need automatically generate the `videocall_location` so that in the confirmation email received by customers already has a link to join. For building this URL, we need to use 'DISCUSS_ROUTE' when appointment is being created (see the linked ENT PR). For making 'DISCUSS_ROUTE' easily accessible, instead of keeping it as a global variable, we now make it a class property of the model `calendar.event`. taskID-2821957 closes odoo/odoo#92628 Related: odoo/enterprise#27136 Related: odoo/upgrade#3558 Signed-off-by: Warnon Aurélien (awa) <awa@odoo.com>