[Tails-dev] review and merge feature/6999-compress-png

Thanks to the investigation of naar on PNG compression, I'm submitting a
branch that compresses the PNG images we ship from the website in our
ISO and adapt the release process accordingly.

ticket:    https://labs.riseup.net/code/issues/6999
branch:    feature/6999-compress-png
milestone: 1.3.1