Adds references between various group related tables.


git-svn-id: https://svn.calendarserver.org/repository/calendarserver@13360 e27351fd-9f3e-4f54-a53b-843176b1656c
fix txdav.who.test.test_group_attendees.GroupAttendeeReconciliation.test_groupRemovalFromDirectory test (still fails)


git-svn-id: https://svn.calendarserver.org/repository/calendarserver@13361 e27351fd-9f3e-4f54-a53b-843176b1656c