Admin fix

This PR covers 2 "admin" tickets

To test  (see http://trac.openmicroscopy.org.uk/ome/ticket/11818)
 * Log as Will (Admin)
 * Go to the admin tab
 * Go to Swedlow lab.
 * Expand the group (known limitation)
 * Right-click and select "Add existing user..."
 * Will should be able to remove and re-add himself from the group
 * Now go to the System group. Root should be greyed out. It should not be possible to remove it.

Second test: (see #http://trac.openmicroscopy.org.uk/ome/ticket/11838)
 * log in as a non-root user (if testing locally, create a new group and a new user in it),
 * in Insight - click on the user name in the treeviewer so that right hand panel shows user info,
  * in the group drop-down the "user" group, should not be displayed.