Metadata lookup

Many times people just download a PDF and want to automatically retrieve the metadata (say, on Zotero import). It would be cool to provide a service mapping md5sum(pdf) → canonical URL. We could implement this even in our static context just by creating a directory of symlinks from https://aclweb.org/anthology/hash/X → https://aclweb.org/anthology/Y. This could be computed separately in the same way we currently build paper thumbnails.