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/nijie.rb

@@ -53,8 +53,8 @@ module Sources
       FILENAME = %r!(?:(?<illust_id>\d+)_(?<page>\d+_))?(?<artist_id>\d+)_(?<timestamp>\d{14})(?:_\d+)?!i
       IMAGE_URL = %r!\Ahttps?://pic\d+\.nijie\.info/#{DIR}/#{FILENAME}\.\w+\z!i
 
-      def self.match?(*urls)
-        urls.compact.any? { |x| x.match?(BASE_URL) }
+      def domains
+        ["nijie.info"]
       end
 
       def site_name