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/sources/strategies/pixiv.rb

@@ -88,16 +88,12 @@ module Sources
       def match?
         return false if parsed_url.nil?
         return false if url.include? "/fanbox/"
+        return false if Source::URL::PixivSketch === parsed_url
         parsed_url.domain.in?(domains) || parsed_url.host == "tc-pximg01.techorus-cdn.com"
       end
 
       def site_name
-        # XXX pixiv sketch should be in a separate strategy
-        if parsed_url.host.in?(%w[sketch.pixiv.net img-sketch.pixiv.net img-sketch.pximg.net])
-          "Pixiv Sketch"
-        else
-          "Pixiv"
-        end
+        "Pixiv"
       end
 
       def image_urls