feat(css): Add utility classes

What does this MR do?

As part of working on customers-gitlab-com#2005 (closed), I've realized I added some wrong utility classes to gitlab-ui, classes which I intend to remove and replace with the correct ones.

Added classes

• gl-pt-12
• gl-md-pt-12
• gl-pb-11

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.