add gunicorn/nginx docker image

adds a generic gunicorn+nginx container

#### What's this PR do?
- builds on the `texastribune/supervisor` container by adding `gunicorn` and `nginx`
- adds configuration files for nginx, gunicorn
- adds supervisord configuration files for both

#### Why am I doing this?  How does this help us?
- should make it easier to deploy any generic Django/WSGI application as long as a couple of requirements are met
- Newsapps or others can concentrate on Django code and worry less about production deployments 
- simplifies descendent Dockerfiles: they can remove boilerplate
- simplified projects: can remove 4 config files: gunicorn, nginx, and their supervisor files

#### How should this be manually tested?
- check out this repo and build: `make`
- check out the `new-docker-container` branch of https://github.com/texastribune/salaries.texastribune.org
- run it with Docker:
```
make build
make db
make es
make run
```
- run it via `runserver`: `python manage.py runserver 0.0.0.0:8000`

#### Next steps:
- if this looks good then test and merge related branch on salaries
- tag these images so descendants can "pin" images to certain versions?
- convert the texastribune image to use it
- convert tribtalk to use it
- dockerfy localdebt with it
- consider splitting into two images: one for nginx and one for gunicorn, but for now this seems simpler

#### Smells?
- standardizes placement of `wsgi.py`, `manage.py`; that may not be palatable to all
- subsequent changes to this repository should be done with caution as they will affect descendent images