CONTRIBUTING.md misleading link to guide

doc/index.rst : more readable Issue Tracker's link on sidebar

Remove mesos lib from pyspark-notebook

upgrade jupyterlab to 2.1.3 to avoid npm delay

The best links connects a linking object and its text occurrence, e.g. the link to documentation is near documentation's occurrence in the text. Since no explicit link to the guide the guide text font became bold but link to the docs moved to doc's text occurence

Print image size after building it

Updating Power PC LE Dockerfile

Fix #1077

Use pip --no-cache-dir to install tensorflow

Add a recipe for auto_sklearn

Updating miniconda version to 4.8.2