feat(css): Add -gl-mb-n8 util class

mentioned in commit 8bb120b9

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!201 (merged)

mentioned in merge request status-page!867 (merged)

mentioned in merge request gitlab!130501 (merged)

mentioned in merge request gitlab-docs!4218 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3521 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!203 (closed)