Implement logged-out top header for Super Sidebar

What does this MR do and why?

Implement logged-out top header for Super Sidebar

This is the first iteration of the top-header for the super sidebar. See: #389603 (closed)

The hover-colors on the register-button are not perfect yet.

Screenshots or screen recordings

Before	After (.com)	After (self-managed)

How to set up and validate locally

1. echo "Feature.enable(:super_sidebar_logged_out)" | rails c
2. In an igcognito window (or logged out) go to any public GitLab route e.g. /flightjs/Flight/-/merge_requests/4

MR acceptance checklist

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