Create a stubbed super sidebar

What does this MR do and why?

Creates a super sidebar stub. This only implements basic UI elements and styles that will then be plugged-in with actual data in follow-up MRs.

Screenshots or screen recordings

How to set up and validate locally

1. Enable the feature flag:

echo "Feature.enable(:super_sidebar_nav)" | rails c

2. Enable the user setting from the user dropdown:
3. The super sidebar should now appear on every page.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #385095 (closed)