Using system Python 3 distribution (3.7.3) in order to allow the use of Debian packages for numpy, scipy, etc. without the need of building them or installing build dependencies.
Changed `#!/bin/sh` in shell scripts to `#!/bin/bash` to make `set -o pipefail` work.
- Change celery app to not be a Django app, more like a WSGI app
- Define a Celery task in the Django users app
- Write a test to execute the task
- Update scripts to use the new app to start workers
- Update documentation
Fix#865
