Implement our masterplan about fallback DNS round-robin pool & HTTPS

Originally created by @Anonymous on #12833 (Redmine)

see https://labs.riseup.net/code/issues/9796#note-25

Blueprint: https://tails.boum.org/blueprint/HTTP_mirror_pool/#HTTPS

Feature Branch: 451f/mirror-pool-dispatcher.git/master and 451f:tails.git/feature/12833+use_wikimedia_mirror

Related issues

• Related to #9796 (closed)
• Blocked by #15908 (closed)