Fix #4260: Unable to replace cdn.donmai.us images?

Bug: Replacing posts hosted on cdn.donmai.us didn't work.

Cause: Original files on cdn.donmai.us are hosted under /var/www/danbooru/original/, but replacements
were trying to store them directly under /var/www/danbooru, which failed with a permission error.
We were trying to store them in the wrong directory because we didn't respect the `original_subdir`
option when generating file paths.

app/logical/storage_manager.rb

@@ -85,7 +85,7 @@ class StorageManager
     when :large
       "#{base_dir}/sample/#{subdir}#{file}"
     when :original
-      "#{base_dir}/#{subdir}#{file}"
+      "#{base_dir}/#{original_subdir}#{subdir}#{file}"
     end
   end