🐛 Fixed broken assets for theme/design preview

refs https://github.com/TryGhost/Team/issues/1190

- The assets were broken in Admin when the frontend and admin urls were different
- Fixed the issue by changing the `asset` helper to output absolute URLs when the frontend/admin urls are differents

core/frontend/helpers/asset.js

@@ -2,7 +2,7 @@
 // Usage: `{{asset "css/screen.css"}}`
 //
 // Returns the path to the specified asset.
-const {metaData} = require('../services/proxy');
+const {metaData, urlUtils} = require('../services/proxy');
 const {SafeString} = require('../services/rendering');
 
 const errors = require('@tryghost/errors');
@@ -22,6 +22,14 @@ module.exports = function asset(path, options) {
             message: tpl(messages.pathIsRequired)
         });
     }
+    if (typeof urlUtils.getSiteUrl() !== 'undefined'
+            && typeof urlUtils.getAdminUrl() !== 'undefined'
+            && urlUtils.getSiteUrl() !== urlUtils.getAdminUrl()) {
+        const target = new URL(getAssetUrl(path, hasMinFile), urlUtils.getSiteUrl());
+        return new SafeString(
+            target.href
+        );
+    }
 
     return new SafeString(
         getAssetUrl(path, hasMinFile)

test/unit/frontend/helpers/asset.test.js

@@ -75,4 +75,27 @@ describe('{{asset}} helper', function () {
             String(rendered).should.equal('/assets/js/asset.min.js?v=abc');
         });
     });
+
+    describe('different admin and site urls', function () {
+        before(function () {
+            configUtils.set({url: 'http://127.0.0.1'});
+            configUtils.set({'admin:url': 'http://localhost'});
+        });
+
+        after(function () {
+            configUtils.restore();
+        });
+
+        it('handles favicon correctly', function () {
+            rendered = asset('favicon.ico');
+            should.exist(rendered);
+            String(rendered).should.equal('http://127.0.0.1/favicon.ico');
+        });
+
+        it('handles ghost.css for default templates correctly', function () {
+            rendered = asset('public/ghost.css');
+            should.exist(rendered);
+            String(rendered).should.equal('http://127.0.0.1/public/ghost.css?v=abc');
+        });
+    });
 });