Top level Navigation, search and hierarchy merger -> Design spinoff

This is not a real issue, just a showcase to show some designs which spin off the work of https://gitlab.com/gitlab-org/gitlab-ce/issues/28285 and https://docs.google.com/document/d/1xeleY5mQhfXpPGMbTnlqbjH90WGsaG6jaP9pmk75PUs/edit

  1. 1_a 1_b
  2. 2_a 2_b
  3. 3_a 3_b