OMERO5DEV__3: fileset image linkage

As discussed recently, all images should link directly to
the Fileset that created them (or null if synthetic). Plate
no longer has a direct link to the Image, which simplies
several queries.

Import should continue to work and delete and chgrp for
images/filesets should all still work with the same
caveats for all multi-file images as previously.

Note: fstest/model.py still needs minor changes but
that conflicts with gh-941.