ExternalData: Convert content links from MD5 to SHA512

Populate a script with `map_<md5>=<sha512>` variable settings and
then run the following to convert the content link files:

    git ls-files -- '*.md5' | while read f; do
      md5="$(cat "$f")"
      sha512="$(eval echo \${map_$md5})"
      rm "$f"
      echo $sha512 > ${f%.md5}.sha512
    done