Migrate to JupyterLab3

We should upgrade the user image to use JupyterLab3.
* Original issue :arrow_right:  https://github.com/utoronto-2i2c/jupyterhub-deploy/issues/81
* Existing PR addressing this in the old utoronto deployment repository :arrow_right: https://github.com/utoronto-2i2c/jupyterhub-deploy/pull/98
* Freshdesk ticket request :arrow_right: https://2i2c.freshdesk.com/a/tickets/57