18.0 [FIX][IMP] calendar_import_ics: fix date parsing, add attendees by email

[len-foss]

Three things:

1. you need to be in the hidden group to access the feature. It automatically adds the admin user, and adds a usage mention.
2. when testing by importing from another Odoo, a number of entries didn't have the "Z" at the end of the DTSTART and DTEND. This would just crash the whole import
3. you can select a number of partners, which are used to be added as attendee if their email is found in the event info. It's a tad complex but gives full control over the import.

The third point is a bit iffy regarding current functionality; before, you can import the whole file, and if you reimport it with an empty date range, it would remove all imported events. If you imported other attendees, the unlink won't run. Since we have the exact event uid, we could add another boolean control. I'm not sure what is the exact use case for this feature, so I'm not sure what would be the best course of action on that.