moebooru: delegate to substrategy based on post source (#3911).

If the yande.re or konachan.com post has a source from a supported site,
for example Pixiv or Twitter, then delegate the artist and commentary
lookup to that substrategy.

Only do this for sources from recognized sites, not the null strategy.

app/logical/sources/strategies/null.rb

@@ -1,10 +1,6 @@
 module Sources
   module Strategies
     class Null < Base
-      def self.match?(*urls)
-        true
-      end
-
       def image_urls
         [url]
       end