Add attendees to the ics file

[davinotdavid]

Description of the Change

• Updates the PROID to -//thunderbird.net/Thunderbird Appointment//EN

• If there's a slot attendee, it adds:

  • MAILTO as the attendee email
  • CN as the attendee name if present or email if not
  • PARTSTAT as CONFIRMED [per RFC iCalendar spec]
  • ROLE as REQ-PARTICIPANT

• HOLD event example (before confirmation):

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//thunderbird.net/Thunderbird Appointment//EN
METHOD:REQUEST
BEGIN:VEVENT
SUMMARY:HOLD: Appointment - Davi Nakano and John McAttendee
DTSTART:20260109T210000Z
DTEND:20260109T213000Z
DTSTAMP:20260108T191025Z
UID:d5f3284e73b4476c885210b8e8f5e60d
ATTENDEE;CN="John McAttendee";PARTSTAT=ACCEPTED;ROLE=REQ-PARTICIPANT:MAILT
 O:[email protected]
DESCRIPTION:None
ORGANIZER;CN=dnakano;ROLE=CHAIR:MAILTO:dnakano
STATUS:TENTATIVE
END:VEVENT
END:VCALENDAR

• Confirmed event example (after confirmation):

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//thunderbird.net/Thunderbird Appointment//EN
METHOD:REQUEST
BEGIN:VEVENT
SUMMARY:Appointment - Davi Nakano and John McAttendee
DTSTART:20260109T210000Z
DTEND:20260109T213000Z
DTSTAMP:20260108T191049Z
UID:d5f3284e73b4476c885210b8e8f5e60d
ATTENDEE;CN="John McAttendee";PARTSTAT=ACCEPTED;ROLE=REQ-PARTICIPANT:MAILT
 O:[email protected]
DESCRIPTION:None
ORGANIZER;CN=dnakano;ROLE=CHAIR:MAILTO:dnakano
STATUS:CONFIRMED
END:VEVENT
END:VCALENDAR

Benefits

• Better formatted ics event with explicit attendees following the iCalendar RFC

Applicable Issues

Fixes #1003

[devmount]

Works on my end. Thanks!