text: Title: Disable some of the extensions by default
  Body: We should think more carefully about which extensions are available by default. One option is to disable the Assignment List (which requires the exchange to be properly setup) and the Courses extension (which is really only used for JupyterHub) and only keep the Formgrader, Create Assignment, and Validate extensions. Then in the installation docs (both in the user and development docs) we can indicate that if people want to install these extensions they should follow the instructions for a server-based setup.