[caldav] Don't send events already deleted on the server, and handle add/modification conflicts

Discard local changes for incidences already deleted remotely.

Also if an event was received from the server then modified locally
before the next sync, ensure it is sent as a modification rather than
an addition.
[caldav] Don't copy the organizer as an attendee

ExtendedCalendar automatically copies this data, but we don't want this
behaviour as it turns the event into an invitation for the organizer.
[caldav] Ignore a remote change if it was locally deleted already.