Copied from the feature/15281-single-squashfs-diff branch in iuk.git
at commit abe82504d03bacbc3e803c9a9d53d4f359488239.

On top of that:

 - Removed PO files as translations will be managed with tails.git's system
 - Removed obsolete PO and Git integration
 - Moved most dependency management from dist.ini to our "release" doc