Add a method to get a pointer to the graph from the graph manager.

This will allow us to just pass around the GraphManager when it's
unlikely we'll need a pointer to full graph (and if we do it's just
an unsafe cast away.)
Implemented separate CRUD for DocumentaryUnit descriptions.

The CRUD views have been extended so they can handle creating/
updating/deleting dependent items with the permissions check being
on the parent.

TODO: Generize this behaviour somehow.