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/twitter.rb

@@ -9,10 +9,6 @@ module Sources::Strategies
     # https://developer.twitter.com/en/docs/developer-utilities/configuration/api-reference/get-help-configuration
     RESERVED_USERNAMES = %w[home i intent search]
 
-    def self.match?(*urls)
-      urls.compact.any? { |x| x =~ PAGE || x =~ ASSET}
-    end
-
     def self.enabled?
       TwitterService.new.enabled?
     end
@@ -35,6 +31,10 @@ module Sources::Strategies
       end
     end
 
+    def domains
+      ["twitter.com", "twimg.com"]
+    end
+
     def site_name
       "Twitter"
     end