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

UID env is not set when launching as root, use `id -u` check instead

Upgrade Spark to 2.2.0

Enable master branch build-tag-push on Docker Cloud

Must be located in the root of each docker build context
i.e., every stack folder

miniconda 4.3.x, python 3.6, spylon 0.4, sparklyr

[Update] Spark updated to 2.1.1

Revert "Fix sudoers.d inclusion on debian img (fix #394)"

This reverts commit 4b3b6697.

Add logs to start.sh

Fix sudoers.d inclusion with debian img (fix #394)

singleuser-start adjustments