Add a method for converting a source URL into a profile URL. This will
be used for normalizing profile URLs in artist entries.
Also add the ability to parse a few more profile URL formats.
* Fix error when uploading non-ugoira files.
* Fix sample image URLs not being rewritten to full images correctly. We
have to get the full image URL from the API because given an
/img-master/ URL, we don't know what the original file extension is.
* Drop support for preview_urls. This means that IQDB lookups may be
slower, especially for ugoiras, since we have to download the full
ugoira now. However, ugoira lookups should produce better results,
since the ugoira thumbnail chosen by Pixiv wasn't necessarily the same
as the thumbnail chosen by Danbooru.
* Drop support for uploading single manga pages:
http://www.pixiv.net/member_illust.php?mode=manga_big&illust_id=18557054&page=2
Previously uploading an URL like this would only upload a single image
out of a multi-image work. Now it will upload all images in the work.
Pixiv no longer supports URLs like this, so we don't either.
* Add support for parsing URLs like this:
https://i.pximg.net/c/360x360_70/custom-thumb/img/2022/03/08/00/00/56/96755248_p0_custom1200.jpg
Apparently artists can choose a custom thumbnail now (not like anyone
will try to upload one though).
