Multicontents

- Try import `transutils` from either `jupyter-server` (newer version) or `notebook` (old version)

- Support for newer version of `jupyter-server`

- Add MultiVersionsCheckpoints (the GenericFileCheckpoints only allow you store single version)
- Set the default of MulitContentsManager to use MultiVersionsCheckpoints
- Recursive rename contents cross content managers, previously we only move the top layer contents.

- import notebook.transutils to avoid issue of "name '_' is not defined"

- avoid user renaming the virtual directory

- initial version