Flag meme redirect as external

Summary: Fixes T5918.

Test Plan: Verified memes work again.

Reviewers: hach-que, btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5918

Differential Revision: https://secure.phabricator.com/D10307

src/applications/macro/controller/PhabricatorMacroMemeController.php

@@ -15,7 +15,9 @@ final class PhabricatorMacroMemeController
     if ($uri === false) {
       return new Aphront404Response();
     }
-    return id(new AphrontRedirectResponse())->setURI($uri);
+    return id(new AphrontRedirectResponse())
+      ->setIsExternal(true)
+      ->setURI($uri);
   }
 
   public static function generateMacro($user, $macro_name, $upper_text,