[jan] Display CalDAV URL of system calendars and tasklists (Bug #12342).
[jan] Display CalDAV URL of system tasklists (Bug #12342).
Revert "There is no group support in Horde_Dav yet, and if there was, then this wouldn't be a group."

Use this workaround until we implement sharing: https://github.com/fruux/sabre-dav/blob/1.8/lib/Sabre/CalDAV/Backend/SharingSupport.php

This reverts commit 87739cc3f4fea269bc488a637f845cac7337c393.