fixed: running gunicorn #4; cleans some other things

    `dash` shell does support `source`-ing but only with a `.`

        modified:   roles/celery/templates/celery_start.j2
        modified:   roles/celery/templates/celerycam_start.j2

    ansible's `django_manage` module is superior to a custom command

        modified:   roles/web/tasks/setup_django_app.yml

    ansible `supervisorctl` module is superior to a custom command

        modified:   roles/web/tasks/setup_supervisor.yml

    ensure `gunicorn` is installed

        modified:   roles/web/tasks/setup_virtualenv.yml

    the actual fix to #4, plus using dash, -2 Mb footprint so far

        modified:   roles/web/templates/gunicorn_start.j2

    templated environment variables so as to only update in one place

        modified:   roles/web/templates/virtualenv_postactivate.j2

    required `django_environment` vars dict to use `django_manage`

        modified:   roles/web/vars/main.yml