Removed babel polyfill

no issue
- the polyfill is no longer required for latest browsers (tested on Chrome, FF, and Safari)
- Edge may have problems but it's not currently supported
- reduces build size. Before/after:
  - `vendor.min.js: 3.29 MB (706 KB gzipped)`
  - `vendor.min.js: 3.2 MB (672.92 KB gzipped)`

ghost/admin/ember-cli-build.js

@@ -102,7 +102,7 @@ module.exports = function (defaults) {
     let app = new EmberApp(defaults, {
         'ember-cli-babel': {
             optional: ['es6.spec.symbols'],
-            includePolyfill: true
+            includePolyfill: false
         },
         'ember-composable-helpers': {
             only: ['toggle']

ghost/admin/lib/koenig-editor/addon/components/koenig-editor.js

@@ -221,8 +221,8 @@ export default Component.extend({
         // add our default atoms and cards, we want the defaults to be first so
         // that they can be overridden by any passed-in atoms or cards.
         // Use Array.concat to avoid modifying any passed in array references
-        atoms = Array.concat(defaultAtoms, atoms);
-        cards = Array.concat(defaultCards, cards);
+        atoms = defaultAtoms.concat(atoms);
+        cards = defaultCards.concat(cards);
 
         return assign({
             placeholder: this.placeholder,