Configured auto-imported assets to load from supplied URL

refs https://github.com/TryGhost/DevOps/issues/47

- this allows us to optionally configure a URL to set as the CDN URL in
  Admin

ghost/admin/ember-cli-build.js

@@ -105,6 +105,16 @@ if (process.env.CI) {
     denylist.push('ember-cli-eslint');
 }
 
+let publicAssetURL;
+
+if (isTesting) {
+    publicAssetURL = undefined;
+} else if (process.env.GHOST_CDN_URL) {
+    publicAssetURL = process.env.GHOST_CDN_URL + 'assets/';
+} else {
+    publicAssetURL = 'assets/';
+}
+
 module.exports = function (defaults) {
     let app = new EmberApp(defaults, {
         addons: {denylist},
@@ -233,7 +243,7 @@ module.exports = function (defaults) {
             }
         },
         autoImport: {
-            publicAssetURL: isTesting ? undefined : 'assets/',
+            publicAssetURL,
             webpack: {
                 resolve: {
                     fallback: {