complete support for Time Zones by Reference (RFC7809)
In #166 (closed) Andrew mentioned we "probably" support Time Zones by Reference, but it may be incomplete and we don't advertise our support in response to an OPTIONS request, which would be mandatory (3.1.1, "calendar-no-timezone").
This issue is a reminder to investigate what's there and what's still needed to implement the RFC. Also, caldav-tester has a CalDAV/timezones.xml suite with some tests requiring the timezones-by-reference feature, which may (or may not) be helpful.