Added support for prepending CDN URLs to Admin assets

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

- this allows us to produce builds that use URLs from a CDN if the env
  var is provided

ghost/admin/ember-cli-build.js

@@ -135,6 +135,7 @@ module.exports = function (defaults) {
         },
         fingerprint: {
             enabled: isProduction,
+            prepend: process.env.GHOST_CDN_URL || '',
             extensions: [
                 'js',
                 'css',