[IMP] calendar: make 'DISCUSS_ROUTE' a class property

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>