Add celery worker to backend-app-1
Replace celery with path to celery in venv

This should allow starting celery without an active venv
Don't manage celery worker process in development
Refactor app resource class

This creates a new resource class, python_app, which the gunicorn_app
resource now uses.

This means that python_app can be used without implying a web app, e.g.
for a worker daemon.
Merge pull request #669 from alphagov/add-celery-worker

Add celery worker to backend-app