🐛 fix editor fenced code block handling (#781)
🐛 fix editor fenced code block handling (#781)

closes https://github.com/TryGhost/Ghost/issues/8695
- non-spellchecker mode should be "gfm" not "markdown" so that fenced
code blocks are displayed correctly and other markdown behaviour better
matches the preview/rendering
TryGhost/Ghost-Admin,kevinansfield/Ghost-Admin,acburdine/Ghost-Admin,acburdine/Ghost-Admin,kevinansfield/Ghost-Admin,TryGhost/Ghost-Admin