Allow suggestions to be copied and pasted as GFM

Depends on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22797.

What does this MR do?

Ensures that Merge Request suggestions can be copied from a note, and pasted into Markdown fields, with the pasted GFM looking exactly like the GFM that generated the suggestion.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• 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

/cc @jerasmus