Single group selection

see https://trac.openmicroscopy.org.uk/ome/ticket/11978

To test:
* Log as user-1 (only one user)
* Make sure you cannot unselect the group item.
* Select "Show All users"
* Go back to the datamanager.
* Click again on display. The "Show All Users" should be selected"
* Unselect, The user currently logged in should still be selected.