Fix working directory problems with custom NB_USER

Fix inherit-diagram: debian@SHA -> ubuntu@SHA

Based on https://github.com/jupyter/docker-stacks/pull/428/commits/0351f0c3b7743026e2aec161b8e23a4182408c74

Add keras

Improve start.sh logging along the way

Update ipywidgets to 7.0.x

[ci skip] Remove py2 reference

use groups for managing write-access to files

it’s handled internally

use `users` instead

ensures files have the right owner:group

unfortunately, not enough to get group-writable permissions (need acl or umask for that),
so we still need to run it after each install

- any files the user should be able to write should have group `user-permissions` with `g+rwX`
- remove `chown` from start.sh because it is no longer needed
- add `fix-permissions` script for setting the user-writable permissions on a path
- user-permissions group as GID 10000 (is there a reason for it to have a different value?)
- containers can set group with `--group-add user-writable` if they want to run with a different uid/gid
  (without -u root -e NB_UID -e NB_GID, which make this unnecessary)

[ci skip] Documentation improvements

Update jupyterlab to 0.26.*

[ci skip] Note in README about Python 2.x

fix condition-testing syntax bugs

make jupyterhub env->cli args optional

Remove Python 2

Only Python 3 from now on

DockerSpawner will stop passing these args,
so make them optional.

proposal: base image on ubuntu:16.04 (LTS)

warn about unused config

update home folder and username on startup

if some env vars are set that aren't used,
complain about it so behaviour is a bit less surprising

closes #414

start.sh: fix check for if we are root