sources: factor out Source::URL::Nijie.

Also fixes the uploader uploading all images when trying to upload only a
single image in a multi-image work. Caused by `image_urls` incorrectly
returning all images when the source strategy was given a url for a
single image.

app/logical/source/url.rb

@@ -25,6 +25,7 @@ module Source
       Source::URL::Lofter,
       Source::URL::Mastodon,
       Source::URL::Moebooru,
+      Source::URL::Nijie,
       Source::URL::Newgrounds,
       Source::URL::Plurk,
       Source::URL::Skeb,