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

#### 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


Why did you have to change the location.  Does the Flask app require that its WSGI file be in a certain subdirectory? (I've not played with Flask before.)  

If this won't work out-of-the-box with other non-Django WSGI apps maybe I should make it clear that's it's only intended for Django and not generic WSGI.