Adjust the release process: downloads are now done using Mirrorbits

Status: !1020 (merged) did part of this.

• wrt. Upgrader changes, see the "Wait until all mirrors in the DNS pool have the new version:" part. Once !983 (merged) is part of the previous version of Tails we shall adjust this. We still have to wait for something, but the DNS pool is irrelevant. ⇒ it now reads _Wait until the IUK start being available-.
• wrt. downloads from our websites:
  • in "Wait for the HTTP mirrors to catch up", we don't need to wait for all mirrors anymore; we only need enough fast & reliable mirrors to be up-to-date
  • in "Public testing", we can point to Mirrorbits as soon as 1 fast & reliable mirror is up-to-date