Fix: Serve favicon image always from the main GitLab domain to avoid issues with CORS

What does this MR do?

When the assets (i.e. the favicon image) are served from a different domain a CORS error occurs. This MR fixes this by serving the favicon images from the main domain instead of the asset domain.

Follow-up to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14497.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

The "custom favicon" feature https://gitlab.com/gitlab-org/gitlab-ce/issues/47802 introduced this regression for GitLab instances that serve assets from a different domain

The development of this MR is sponsored by @ siemens (/cc @bufferoverflow).

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/47802