Bug: When uploading a direct Pixiv image URL, we ignored it in favor of the
image URL returned by the Pixiv API. This meant if you tried to upload the
original version of a revised image, we would get the revised version instead.
Fix: When given a direct Pixiv image URL, use it as-is if it's a full
image URL. If it's a sample image URL, ignore it in favor of the full image
URL as returned by the API, unless the post is deleted and the API data
is unavailable.
