Add spacing utilities
What does this MR do?
This adds the following spacing utilities:
gl-mb-11
gl-p-9
gl-p-13
gl-pt-13
gl-pb-9
gl-pb-13
These will be used in design.gitlab.com
as we migrate custom utils to GitLab UI ones as part of gitlab-org/gitlab-services/design.gitlab.com#372 (closed).