Regular pixiv URLs don't have enough information in the URL to derive the
image's URL, so we use the OpenGraph headers for extracting it.
Since we can't block or the UI freezes, we do the request as part of
Pixiv URLs also need a bit of special treatment. Clicking the thumbnail would
open the direct link to the image in the browser, which isn't very useful because
you would get a 403 back (or a small image), so we inject a link to the web
interface for the image.