sources: factor out Source::URL::ArtStation.

app/logical/source/url.rb

@@ -19,6 +19,8 @@ module Source
   class URL < Danbooru::URL
     SUBCLASSES = [
       Source::URL::Twitter,
+      Source::URL::ArtStation,
+      Source::URL::Foundation,
       Source::URL::HentaiFoundry,
       Source::URL::Lofter,
       Source::URL::Mastodon,
@@ -27,7 +29,6 @@ module Source
       Source::URL::Plurk,
       Source::URL::Skeb,
       Source::URL::TwitPic,
-      Source::URL::Foundation,
     ]
 
     # Parse a URL into a subclass of Source::URL, or raise an exception if the URL is not a valid HTTP or HTTPS URL.