Archive links are invalid: need to fix http://esug.org/data

The ESUG archive (e.g. slides of conferences) used to be 

http://esug.org/data

With the new website, we lost that. And it is no that simple to fix

- this is ~40GB, so we can not put it on github pages
- gitbub pages seemingly can not rewrite URLs

The archive for now is available as

http://old.esug.org/data/