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