Style popovers according to the design specs

Style popovers according to our UI design specs as specified in gitlab-ui#250 (closed)

This MR does not audit the application to enforce the styling of the design system. It adds correct styling for the component added to gitlab-ui.

Follow up issues

• Style implemented popovers according to design system: https://gitlab.com/gitlab-org/gitlab-ce/issues/62814
• Update slot content example in gitlab-ui to be real example gitlab-ui#279 (closed)

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Tested in all supported browsers