Update check-mirrors to support the new mirror pool design
Originally created by @intrigeri on #11122 (Redmine)
It’ll need to deal with the fact tha will will actually two (possibly intersecting?) pools: the remaining DNS one, and the new one described on the blueprint.
Blueprint: https://tails.boum.org/blueprint/HTTP_mirror_pool/#configuration
Feature Branch: check-mirrors:master
Parent Task: #7161 (closed)
Edited by intrigeri