fix #12362, #12396: allow group owner to change permission level (rebased from dev_5_0)

Fixes http://trac.openmicroscopy.org.uk/ome/ticket/12362 and http://trac.openmicroscopy.org.uk/ome/ticket/12396 so that group owners can change group permissions in Insight and get the usual warning if they try to downgrade to Private.

--rebased-from #2655