uploads: add new source:<url> dupe check (fix #3873)

* On the /uploads/new page, instead of just showing a "This post has
probably already been uploaded" message, show the actual thumbnails of
posts having the same source as what the user is trying to upload.

* Move the iqdb results section up top, beside the related posts section.

app/logical/upload_service/controller_helper.rb

@@ -23,11 +23,5 @@ class UploadService
 
       return [upload]
     end
-
-    def self.batch(url, ref = nil)
-      if url
-        return Sources::Strategies.find(url, ref)
-      end
-    end
   end
 end