Decoupling of dashboard and nbgrader

In the actual !126 (merged), the instructor dashboard uses JupyterCourse's generate_assignment, which uses nbgrader to remove solutions from notebooks. This PR intends to get an instructor dashboard using a simple Course class, so as to get rid of any JupyterCourse method that uses nbgrader. It is linked to !127 (closed) . It works on my laptop, but I had difficulties with circular import between course.py and dashboards.py because typing needs them. It might not be necessary for travo 1.0 but maybe for the upcoming first tutorial session @marcopasi @nthiery @corentinmorice1 @cmarmo