Per event permission are currently not taken care of, which means you can
modify meetings even though you are not the event organizer via WebDAV. Normally, only the event organizer can modify the event, but WebDAV modification is an all or nothing event.
Fixing this requires us to somehow hook into the iCalendar import and check what type of modifications is done, and check the access rights for each type of modification for each event. There is also the issue of what to do if you have done several modifications, but only one of them are unallowed. Skip all, or pretend everything is fine, but just not do the unallowed changes?
