Support newer jupyterlab and python-program-analysis

[andrewhead]

This update to jupyterlab incorporates several changes:

• Support newer versions of jupyterlab (at least up until 1.2.1)
• Make nbgather compatible with the updated python-program-analysis. As of last autumn, a few changes were made to python-program-analysis to improve the precision of slicing, though these hadn't yet been incorporated into nbgather.
• Add regression tests to nbgather to make sure that slicing is still behaving as expected. This was added to help us detect changes to python-program-analysis that require updates to nbgather
• Change the test-runner to jest with the intent to enable cleaner and easier regression testing
• Clarify the documentation in the Advanced Setting Editor for user-controllable settings