Fix paste markdown feature in Content Editor

What does this MR do and why?

It fixes pasting Markdown in the Content Editor. When pasting text in the Content Editor and as long as the clipboard’s data only contains a single entry with MIME type text/plain, the Content Editor will transform the Markdown into Rich Text Format.

This MR also ensures that future merge requests won’t break the feature again by adding an integration test that exercises the paste Markdown feature integrating all the intervening components.

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Edit or create a Wiki page in your local environment.
• Copy text from a textarea or editor that fills the clipboard only with text/plain data. You can also paste using Cmd + Shift + V.
• Paste the Content into the Content Editor.
• The pasted Content should be displayed as Rich Text in the Content Editor.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.