Make FreedomBox image downloads available via BitTorrent protocol

Make the stable images of FreedomBox available for download using BitTorrent.

Advantages:

• People with flaky internet connections can reliably download FreedomBox.
• Downloading from peers might be useful at FreedomBox install fests.
• Circumventing censorship of ftp.freedombox.org when deep-packet inspection is used.
• Reduce server congestion and scaling problems when FreedomBox eventually gets popular.
• Other general benefits of decentralization.

Since testing and unstable images are updated very frequently, there is not much advantage to be gained by having multiple bit-torrent seeds which get outdated on a daily basis. Suggestions on what to do about them are welcome.