Overhaul navigation tree on mobile devices

The following discussion from !127 (merged) should be addressed:

• @jovankrunic started a discussion:

  When running on small devices (with tab navigation), the app doesn't offer ways to access pages like:

  1. About
  2. Feedback
  3. Library account
  4. Assessments

  As discussed with @andy.bastian in the meeting yesterday, after @D3CK3R gives his opinion, codeculture will solve this issue in a follow-up rather than in this merge request.

Update: The overview: https://miro.com/app/board/o9J_lihIeT0=/?share_link_id=695402755250