WIP: Fix double bolded markdown in system notes

What does this MR do?

Used HTML to style system notes as bold when wrapping user generated markdown.

Are there points in the code the reviewer needs to double check?

• TODO: Add tests for bold markdown that check the rendered HTML.
• TODO: Add changelog.

Why was this MR needed?

https://gitlab.com/gitlab-org/gitlab-ce/issues/21105

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #21105 (moved)

Closes #31730 (moved)