Deleting a notebook should remove it completely

[predictiple]

Currently (v0.73.3) the Delete Notebook button removes the notebook from the notebooks list but leaves the notebook itself active in the lower pane.

Additionally I think that deleting notebooks should also remove them from disk, since that would probably be in line with most users' expectations. Deleted notebooks that aren't deleted from disk, especially in large deployments, might needlessly waste a lot of disk space. Daily backups will already likely contain most historical notebooks in case recovery is required.