Replace fa-bell icons with GitLab SVG notifications 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-bell

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