Disabled mirage in development

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

config/environment.js

@@ -47,7 +47,7 @@ module.exports = function (environment) {
 
         // Enable mirage here in order to mock API endpoints during development
         ENV['ember-cli-mirage'] = {
-            enabled: true
+            enabled: false
         };
     }