Rework left Menu

As a website user, I want a clearly arranged menu so that I can find my way around the website.

Design: https://projects.invisionapp.com/d/main?origin=v7#/console/21750706/462361966/preview?scrollOffset=579.2000122070312

Acceptance criteria:
- only three level menu (1: Icons, 2: Arrow, 3: only text)
- fourth level on the right side (hide in mobile view)
