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/pixiv.rb

@@ -21,10 +21,6 @@ module Sources
       FANBOX_IMAGE = %r!(?:\Ahttps?://fanbox\.pixiv\.net/images/post/(\d+))!
       FANBOX_PAGE = %r!(?:\Ahttps?://www\.pixiv\.net/fanbox/creator/\d+/post/(\d+))!
 
-      def self.match?(*urls)
-        urls.compact.any? { |x| x.match?(/#{WEB}|#{IMG}|#{I12}|#{TOUCH}|#{PXIMG}|#{FANBOX_IMAGE}|#{FANBOX_ACCOUNT}/i) }
-      end
-
       def self.to_dtext(text)
         if text.nil?
           return nil
@@ -47,6 +43,10 @@ module Sources
         DText.from_html(text)
       end
 
+      def domains
+        ["pixiv.net", "pximg.net"]
+      end
+
       def site_name
         "Pixiv"
       end