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/pawoo.rb

@@ -2,10 +2,8 @@ module Sources::Strategies
   class Pawoo < Base
     IMAGE = %r!\Ahttps?://img\.pawoo\.net/media_attachments/files/(\d+/\d+/\d+)!
 
-    def self.match?(*urls)
-      urls.compact.any? do |x| 
-        x =~ IMAGE || PawooApiClient::Status.is_match?(x) || PawooApiClient::Account.is_match?(x)
-      end
+    def domains
+      ["pawoo.net"]
     end
 
     def site_name