Resolve "Clicking markdown 'Preview' tab spawns two XHR requests"

What does this MR do?

Prevents clicking the "Preview" tab in a markdown field from triggering multiple Ajax requests. Additionally, prevents multiple requests from being generated if "Preview" is clicked more than once before the initial request is complete.

Lastly, refactored some annoying coffeescript-to-javascript conversion cruft and fix some eslint failures.

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

Just ensure clicking preview only triggers one ajax request in chrome devtools.

Does this MR meet the acceptance criteria?

• Changelog entry added
• 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 #25364 (closed)