Logged out marketing header experiment

What does this MR do and why?

This MR implements #346438 (closed)

It introduces an experiment where a user sees a more noticeable (72px tall vs 40px) header with marketing links, separate login and signup links, and some re-arrangements (Menu is on the right):

The experimental header is only shown when no user is logged in and the screen width is sm and above:

With a logged in user: experimental header is NOT shown

On xs screens: header has the same height and layout

Show screenshot

Screenshots or screen recordings

How to set up and validate locally

1. Ensure :logged_out_marketing_header feature flag is enabled

   # In Rails console (run `bin/rails c` in `/path/to/gitlab-development-kit/gitlab`)
   > Feature.enable(:logged_out_marketing_header)

2. Visit a public project without logging in

3. Validate that you can see the experimental header with the correct links

4. Log in

5. Validate that the experimental header is NOT shown

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.