jmap_ical: prefer Cyrus IANA timezones over embedded VTIMEZONEs

Some clients, e.g. Apple implementations, may embed bogus VTIMEZONE
components with valid IANA timezone identifiers in VEVENTs.
Rather than relying on correct IANA timezones being embedded in
iCalendar data, this patch changes Cyrus to prefer its internal
timezones for IANA timezone identifers.