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 May 15, 2020 by intrigeri

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