Replace fa-user(s) icons with GitLab SVG user(s) icon

As part of the effort to replace FontAwesome icons with GitLab SVGs we need to use the warning icon instead of the FontAwesome variants.

3 times: fa-user

4 times: fa-users

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

References

• See https://leipert-projects.gitlab.io/is-gitlab-pretty-yet/icons/ for all Font Awesome icon instances
• See http://gitlab-org.gitlab.io/gitlab-svgs/ for all GitLab icons