markdown editor with live preview.

What does this MR do?

Splits the markdown editor into adjacent Write and Preview panel. The Preview panel is filled periodically with the markdown preview.

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

I'm worried about the frontend performances and user experience with this change.

Also I'm not sure pages with several markdown editors will be handled correctly.

Why was this MR needed?

Switching from Write to Preview is cubersome and tools like dillinger or stackedit prove that a side-by-side preview is a much better experience.

Screenshots (if relevant)

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