Don't select a shared calendar as schedule-default-calendar-URL, fix for #550

Recreating this PR to target 2.1 (see #551).

old description:
>Quite easy to fix as we already have the shared-info in the resource type. As @evert mentioned in #550, I also don't think we really need to check for users having write-access on the calendar as it is currently not possible to create an own read-only calendar (if I didn't miss something).