fix #12011: move/delete datasets that share images

*not ready for review*
Fixes http://trac.openmicroscopy.org.uk/ome/ticket/12011 and assists http://trac.openmicroscopy.org.uk/ome/ticket/11610.

Try moving and deleting datasets that contain images that are also in other datasets. Something not-obviously-wrong ought to happen, which probably suffices until the major graph traversal reworking of https://trac.openmicroscopy.org.uk/ome/ticket/11779 is done. Datasets should be movable and deletable and their images ought not to be left orphaned.

Also check that http://ci.openmicroscopy.org/job/OMERO-5.0-merge-integration-python/lastSuccessfulBuild/testReport/test.gatewaytest/test_chgrp/ and http://ci.openmicroscopy.org/job/OMERO-5.0-merge-integration-python/lastSuccessfulBuild/testReport/test.integration.test_chgrp/TestChgrp/ have no skips or fails.