Release sprint for NixOS 17.09

NixOS 17.09 is going to be released at September 28th, during this meetup we'll try to fix blockers. See the announcement on the nix-devel list (https://groups.google.com/forum/#!topic/nix-devel/vILGXXbeCPg) for more information.

People new to Nix are welcome as well; we'll try to help you understand the big picture, after which you can help us by fixing issues!