Dry up show_super_sidebar? and gon.use_new_navigation helpers

The following discussion from !111186 (merged) should be addressed:

• @leipert started a discussion: (+2 comments)

  @thutterer There is some duplication with https://gitlab.com/gitlab-org/gitlab/-/blob/a00d3c30f217e905037cd8ec8e81fce6d1238aaf/lib/gitlab/gon_helper.rb#L59

  I wonder if we should move show_super_sidebar? to app/models/user_preference.rb maybe? So we could access it with current_user.show_super_sidebar? ?