Draft: Unauthenticated header

What does this MR do and why?

Implements a new logged out header to align with new navigation and replace existing logged out marketing header.

• Create logged_out_header.vue component, styles, and webpack bundle
• Add .with-logged-out-header class when logged out of gitlab.com
• Add --logged-out-header-height conditional CSS custom property added to $calc-application-bars-height scss variable
• Add logged_out_header_context with link data to application helper

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Run branch locally
2. Open incognito browser or logout locally
3. Visit page that displays header e.g. /flightjs/Flight/-/tree/master

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 #389603 (closed)