Tethys-platform

Adds OverviewMap control to Tethys Map View
Adds `set` tag to tethys_gizmos custom tags.

Pin Postgresql dependency to ensure stable environment.

Remove postgis dependency from environment.yml.
The postgis dependency is too restrictive and causes gdal installations to break.

This patch makes the following fixes:
* `tethys docker` CLI API was refactored to work with the latest version of `docker-py`
* upgrade of the `docker-py` version allows the Tethys environment to resolve to Python version 3.6
* fixes to the activate and deactivate script to add the TETHYS_SRC variable to the environment when it is activated

* Python 3 Officially Supported
* Python 2 Support Deprecated
* 100% Unit Test Coverage
* Tethys Extensions
* Many more basemaps on MapView
* Control version of OpenLayers used by MapView
* Class-based controller support
* Partial install options in install script for Linux and Mac
* New commands to automate tethys services and app settings
* Dockerfile
* API Tokens for Users
* Documentation for SSL use in production
* Bug fixes

Fixes an issue with HydroShare OAuth backend that was introduced in 2.0.0.