Disable launcher and new directory in the root directory

[ellisonbg]

The root directory of the jupyter drives is not really a directory, but instead is a list of the buckets. Because of this, creating a file or directory is invalid. When a user tries to do this, in the current version, those operations fail. Right now the error messages are pretty cryptic. At a minimum, we should provide more informative error messages, but ideally, it would be nice to disable the UI in lab for creating files and directories when a user is in the top level of the jupyter drives. This may be a bit subtle though as there are so many paths for creating files, but we may be able to do this by disabling the underlying command. This may require changes to JupyterLab itself, so let's discuss.