Group list from Allow group admins of specific groups to impersonate the users within those groups should be updated if we delete a group in the server

[davitol]

From the PR #134

Steps to reproduce

1 - As admin create 2 groups, populate them and set a subadmin for them
2 - Browse to user authentication menu and add both groups to "Allow group admins of specific groups to impersonate the users within those groups" list
3 - As admin delete one of the groups in oC server
4 - Browse back to user authentication menu and check the list

Expected behaviour

The deleted group should not appear. the list should have been updated

Actual behaviour

The group is shown even it has been deleted in the oC users page

(cc @sharidas FYI )

[PVince81]

similar to #120 and not critical