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.rb

@@ -12,15 +12,12 @@ module Sources
         Strategies::Nijie, 
         Strategies::Pawoo,
         Strategies::Moebooru,
-
-        Strategies::Null # MUST BE LAST!
       ]
     end
 
-    def self.find(url, referer=nil)
-      all
-        .detect { |strategy| strategy.match?(url, referer) }
-        .new(url, referer)
+    def self.find(url, referer=nil, default: Strategies::Null)
+      strategy = all.detect { |strategy| strategy.match?(url, referer) } || default
+      strategy&.new(url, referer)
     end
 
     def self.canonical(url, referer)