Check the validity of download links
We would like to make sure regularly that every download link in the codebase works. The best way to do this is to check automatically with the CI; this also prevents us from merging code with broken links.

This PR adds a small script that:
- Lists all source code files
- Extract links with a regexp
- Perform HEAD requests to check the validity of each links
- Returns an error if at least one link is broken, along with the list of all broken links.

I also add a Circle CI workflow `repository-consistency` with a small machine that runs this script. It could be used to enforce things such as coding styles etc in the future.
It works so well that the CI failed because of a broken link :)