Classification graphs: use exclusivity constraint
Classification graphs: disable restricted sub-class-instance modifications
Classification editor: disable context menu entry all submenu entries are disabled
Ontology: return a QuerySet for get_classes query
Classification graphs: respect restrictions of sub-types, too
Classification: auto-infer class instances based on restricitons
Classification editor: add optional display of super-classes
Classification graphs: child nodes to jsTree structure as separate step
Classification editor: add 'Edit mode' and additional editing tools
Classification editor: remove unneeded code in WM widget
Classification editor: expect every request to be an Ajax request

Unfortunately, the Ajax headers somehow don't get sent with Chrome. So
this change removes the option to render a complete valid HTML page and
not only the content block if the request isn't Ajax based. Those pages
are currently only requested throug Ajax from CATMAID's front-end.
Classification editor: use correct request type and template when adding new graph
Classification editor: speak of graphs instead of trees
Classification graphs: add option to get all graphs not linked to a project
Classification graphs: allow to link an existing graph to an existing project
Classification editor: offer to select a specific graph if more than one linked
Classifiation editor: make sure project and link IDs are of a numeric type
Classification editor: return the link id, too, if editor content is returned
Classification editor: refactor editor content handling and display a selected graph
Classification graphs: create dummy project if not already there