Replace fa-unlink with GitLab SVG unlink icon

As part of the effort to replace FontAwesome icons with GitLab SVGs, this update moves to use the unlink icon in place of the FontAwesome variant.

Current usages of fa-thumbs-up and fa-thumbs-down https://leipert-projects.gitlab.io/is-gitlab-pretty-yet/icons/

1 times: fa-unlink

When you are done

1. Add screenshots from before + after to make review easier and faster
2. Get UX Review
3. Get Frontend Maintainer review directly