Owner remove last user 11984

A couple of small bug-fixes - to test:

- Log in as a group owner, edit one of your groups, removing a user who is ONLY in that group - should get a warning. https://trac.openmicroscopy.org.uk/ome/ticket/11984

- In SPW, select a well and open the 'Preview' tab for the well and click 'Paste' of rendering settings (not necessary to copy settings first, but it wouldn't hurt!). Now double-click on the selected well - only ONE image viewer should open - previously it opened 2. https://trac.openmicroscopy.org.uk/ome/ticket/11987