Fix Markdown preview on New/Edit Release and New Snippet pages

What does this MR do?

Fixes the Preview tab on the New Release page, the Edit Release page, and the New Snippet page.

See #241777 (closed) for details.

GIF

Testing

I added a new feature test to ensure the preview works on the New Release and Edit Release page. I didn't add a test for the New Snippet page because I am less familiar with this page 🤷