document graph traversal reimplementation for developers

Documents the changes to server code brought by https://github.com/openmicroscopy/openmicroscopy/pull/3228; some of the links may remain broken until it is merged. Mostly staged at http://www.openmicroscopy.org/site/support/omero5.1-staging/developers/Server/ObjectGraphs.html.
--no-rebase
why to start talking here about ``our`` images, when the sentence started talking about images of ``a user``.  Also the ``our`` is less precise than ``mine`` in this case.
Maybe: ``For instance, typically I can move only my own objects to a different group, but if another user tags my image with my tag, then I may still move my image and tag to a different group, also moveing that link even though it is not my own object: in that case, permissions checking is disabled for that....``