Fix invalid image helper URLs when using a subdir.

If Ghost is running under a sub-directory, image URLs that are saved include the sub-directory.  The problem is that `config.createUrl()` adds the sub-directory when building URLs, so any time a URL is generated the output will contain the sub-directory twice, e.g. `http://my-ghost-blog.com/subdir/subdir/content/image.png`.

This affects metadata tags that include image URLs and post cover images.