Support CalDAV auto-discovery

[thomaseizinger]

I am using fastmail which uses https://caldav.fastmail.com as their entrypoint for CalDAV. See https://www.fastmail.help/hc/en-us/articles/1500000278342-Server-names-and-ports#h_01H95ZZXYQJCB2E8CEM5PDK8H0 for details.

Plugging that into "Errands" didn't work, it resulted in a 404. Indeed, the above URL returns a 404 when contacted directly. Fastmail makes use of the CalDAV auto-discovery (outlined in https://datatracker.ietf.org/doc/html/rfc6764) which essentially says:

1. Go to <host>/.well-known/caldav
2. Take the re-direct returned (in fastmail's case: https://caldav.fastmail.com/dav/calendars)
3. Use that as a base for caldav operations

Indeed, Errands correctly imports all tasks when the Server URL is set to https://caldav.fastmail.com/dav/calendars