Add integration with Jupyter Checkpoints to the BookstoreContentsArchiver

[mpacer]

We may be able to give people a straightforward way to revert to older versions of their notebooks by integrating directly with the Jupyter Checkpoints APIs.

This would allow us to surface a simple UI for users to revert to older versions of their notebooks from within the notebook itself.

Many thanks to @romain-intel for pointing out this possibility.