Fixed missing CSS styles in production builds

no issue

- some styles such as `border: none` inside `.gh-btn-white` were being stripped when ran through `clean-css` leading to visual differences between development and production
- disabled `ember-cli`'s default CSS minification as we already use `cssnano` for minification in our postcss pipeline for both dev and prod builds

ghost/admin/ember-cli-build.js

@@ -145,6 +145,10 @@ module.exports = function (defaults) {
                 }
             }
         },
+        minifyCSS: {
+            // postcss already handles minification and this was stripping required CSS
+            enabled: false
+        },
         nodeAssets: {
             codemirror: codemirrorAssets(),
             simplemde: simplemdeAssets()