Count downloads from content delivery network

[LaurentRDC]

Following my comment here, we can get reliable download counts despite the existence of a CDN via a redirection scheme:

• Add the Cache-Control: no-store header for the download endpoint;
  • Log the download statistic.
• Return HTTP302 (Redirect) to another endpoint with the current CDN caching configuration.

I am happy to implement this feature.

One point that is unclear is whether this issue should be address after #1342, given that the server will necessarily process more download counts.

[gbaz]

I believe we can use our 302 redirect mechanism for hackage-content here too, so we just would need to turn on redirects to our hackage-content domain for tgz files, with no other work. (And to add the no-store header)