Rename `markdown_preview` routes to `preview_markdown`

Originally opened at !3700 (closed) by @cbartz.

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Closes #15000 (closed).

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together