change sidenav to explicit listing of lectures

This MR is supposed to prepare the scene for adding links to the lectures explicitly in the sidenav. Other merge request might have to rebase once this is merged to adhere to the new protocol. As discussed we want to explicitly list lectures such that

• we can more easily rearrange them
• the navigation menu gets cleaned up

This is still a draft because I just realized that I'll have to update the summary script.