Management command to delete orphaned VersionTrackers

  - Management command to delete orphaned VersionTrackers.
    
    This is useful to clean up deleted pages added while testing.

  - 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.

  - 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.