[IMP] calendar: add discuss video url in calendar

This commit aims to integrate the calendar module with discuss video calls.
Now it is possible to add discuss videocalls by clicking on a button below
the videocall_location field. The discuss videocall_location is a URL that
is computed and it creates a discuss channel only when someone accesses that
URL.

For recurring events, the same discuss channel is used for all events.
However, each event in the recurrency has its own videocall_location URL.
This is done on purpose in order to allow the functionality even if the base
event of the recurrency is deleted. For single events, each event will create
a discuss channel.

task-2685472

Part-of: odoo/odoo#79654