Fix the page_url method not to return URLs like this:
https://seiga.nicovideo.jp/image/source/8017978 (page: https://seiga.nicovideo.jp/watch/mg310193)
These are direct image URLs, not page URLs. It's not generally possible
to get to the page URL from an image URL like this.
This fixes it so that we don't incorrectly set the source of NicoSeiga
uploads to the image URL.
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.