HG-332 Linked images

Handle images that link to other articles.

Related Wikia/app change: https://github.com/Wikia/app/pull/5340

https://wikia-inc.atlassian.net/browse/HG-332
But it could be an external link to, or external to another wiki. I'm not sure if it makes sense to use {{#link-to 'article'}} then.