Ics file generation should be possible even when only event is
installed as it is a basic feature of events.
Moreover we need to be able to generate such files in order to replace
the old outlook api used in the event templates, api which is not
supported anymore.
We decided to use ics files because it is supported by most calendar
schedulers hence working with outlook.
Linked to task #1853063closesodoo/odoo#28477
