WIP: Scope application.css .md into .md-legacy

What does this MR do?

• Fix font-size for comments (https://gitlab.com/gitlab-org/csslab/issues/61)
• Fix preview markdown type is inconsistent in csslab integration (https://gitlab.com/gitlab-org/csslab/issues/62)

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer