sources: fix sources sometimes choosing wrong strategy (fix #3968)

Fix sources choosing the wrong strategy when the referer belongs to a
different site (for example, when uploading a twitter post with a pixiv
referer).

* Fix `match?` to only consider the main url, not the referer.

* Change `match?` to match against a list of domains given by the `domains` method.

* Change `match?` to an instance method.

app/logical/sources/strategies/art_station.rb

@@ -9,10 +9,6 @@ module Sources::Strategies
 
     attr_reader :json, :image_urls
 
-    def self.match?(*urls)
-      urls.compact.any? { |x| x.match?(PROJECT) || x.match?(ASSET) || x.match?(PROFILE)}
-    end
-
     # https://www.artstation.com/artwork/04XA4
     # https://www.artstation.com/artwork/cody-from-sf
     # https://sa-dui.artstation.com/projects/DVERn
@@ -24,6 +20,10 @@ module Sources::Strategies
       end
     end
 
+    def domains
+      ["artstation.com"]
+    end
+
     def site_name
       "ArtStation"
     end