Chgrp target 10908

Fix chgrp of images to a target Dataset in new group (also written tests for this). To test:

 - In web, choose all images from a MIF and move to a new group, choosing Dataset target.
 - Check that the move completes and the images end up in the chosen Dataset.
 - Bonus [already tested by @joshmoore] Run the ``` chgrp.py TestChgrpTarget.testChgrpMifImagesToTargetDataset ``` test