epic: Publish new stable builds for Debian Buster

This epic exists only to give a serial order to implementation of tasks.

• Move old builds to an old-stable folder on ftp.freedombox.org
• Publish new stable builds for Debian Buster using Buildbot infrastructure#16 (closed)
• Make BitTorrent files available for download #58
• Announcement on freedomboxfoundation.org and social media