moebooru: proxy images to avoid hotlink protection (#3911).

Fix image previews being broken on the upload page due to yande.re not
allowing hotlinking of images.

app/logical/sources/strategies/moebooru.rb

@@ -76,6 +76,10 @@ module Sources
         nil
       end
 
+      def headers
+        { "Referer" => "http://#{site_name}" }
+      end
+
       # Moebooru returns an empty array when doing an md5:<hash> search for a
       # deleted post. Because of this, api_response may be empty in some cases.
       def api_response