Don't leave orphan contents when deleting.

Deleting a content should delete all its children. Content.delete()
remains the canonical way to delete a widget, it just works better now.
Allow deletion of VersionTrackers

They couldn't be deleted before because of circular foreign keys and
frozen nodes. Now, frozen nodes are overridden -- it is assumed that if
you delete a VersionTracker you know what you are doing, and want to
destroy history.
Management command to delete orphaned VersionTrackers.

This is useful to clean up deleted pages added while testing.