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.
15799f8af7