sources: factor out Source::URL::PixivSketch.

Add upload support for Pixiv Sketch. Fetch tags, commentary, and artist,
and rewrite sample images to full images.

Authentication isn't required. R18 images are hidden in the browser but
visible in the API.

app/logical/source/url.rb

@@ -28,6 +28,7 @@ module Source
       Source::URL::Moebooru,
       Source::URL::Nijie,
       Source::URL::Newgrounds,
+      Source::URL::PixivSketch,
       Source::URL::Plurk,
       Source::URL::Skeb,
       Source::URL::TwitPic,