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/nico_seiga.rb

@@ -7,8 +7,8 @@ module Sources
       PAGE = %r!\Ahttps?://seiga\.nicovideo\.jp/seiga/im(\d+)!i
       PROFILE = %r!\Ahttps?://seiga\.nicovideo\.jp/user/illust/(\d+)!i
 
-      def self.match?(*urls)
-        urls.compact.any? { |x| x.match?(URL) }
+      def domains
+        ["nicoseiga.jp", "nicovideo.jp"]
       end
 
       def site_name