Add support for checking IUKs to check-mirrors

Originally created by @intrigeri on #6484 (Redmine)

Our check-mirrors script should check that all IUKs advertised by upgrade-description files are available (with the correct size and hashsum) on the mirrors.

Source code: git clone https://git.tails.boum.org/check-mirrors
Mentoring: tails-mirrors@boum.org

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information