Track clicks on download links

[alycejenni]

Track clicks on the links generated by the "server" classes in the new download model.

[jrdh]

First thought on how to do this: write a route in CKAN that receives the download requests, update some stats database with the download request, use the NGINX x-accel header to get NGINX to serve the actual file.
Main issue with this is that whichever NGINX needs server gets the header from CKAN needs to be able to serve the file, i.e. needs access to the disk it's stored on. Maybe we could start using object storage in Valentine to share these files?