Add optional, opt-in logging

[andrewhead]

Describe the solution you'd like
It would be great if we could collect usage data from people who are using code gathering tools in their work. This could help us decide on future improvements to this tool, and collect data that could be shared other researchers and tool builders build better notebook tools.

Data collection would be strictly opt-in, and would be off by default.

Additional context
To do this, we would need the following:

• Fill out relevant institutional paperwork
• A non-intrusive, easy-to-dismiss popup that appears when someone uses the tool, that asks if folks want to share their usage data with us, with a description of the data that will be collected, anonymization of the data, and research goals
• Persistent storage of a user's choice. Use this choice to decide whether to report log events
• A server (e.g., notebooks.berkeley.edu) that receives log events over a secure server

Some of the events we might want to collect are:

• when code gathering events get invoked, how large are the slices? Where in the notebook do they come from? Are these slices opened in notebooks, scripts, or as cells in existing notebooks?

[andrewhead]

For an example of a popup in the JupyterLab code, see the "change kernel" popup: https://github.com/jupyterlab/jupyterlab/blob/master/packages/apputils/src/clientsession.tsx#L395. This creates and reads the values set in a Dialog widget.

See here for instructions on persistent state storage for saving a user ID.

May want to add validation for the email field.