Disabled mirage in development
Disabled mirage in development

closes https://github.com/TryGhost/Ghost/issues/10414
- mirage's `passthrough()` was not allowing `/ghost/assets/simplemde/simplemde.js` to hit the backend server which was breaking markdown cards

TryGhost/Ghost-Admin,TryGhost/Ghost-Admin,kevinansfield/Ghost-Admin,kevinansfield/Ghost-Admin