Add Lectures and Assignments to Jupyterlab Menu

[meffmadd]

Is your feature request related to a problem? Please describe.
Users should also be able to access the lecture and assignments list or other functionality in the Jupyterlab menu.
(see: https://github.com/jupyterlab/extension-examples/tree/main/main-menu)

Describe the solution you'd like
A simple menu could be a list of lectures and where each lecture has a submenu with the assignments. We have to change our widgets so that they can accept a path parameter, which describes which lecture/assignment to open. The routing then is handled by react-router-dom.

TODO:

• Implement menus with lectures and assignments in submenus
• Call updateMenus(true) when assignment is added to reload menus
• Fix widget restorer (assignments and course management are no restored when reloading page)