change Dockerfile to use gunicorn, requiring some changes to the app.

[stvhay]

Since i think the intent this is used in production, the Dockerfile is adjusted to use gunicorn rather than the insecure build-in WSGI server Flask uses. This required some changes to some imports in the app itself. Also adjusted the order of commands in the Dockerfile so the layers work out a bit better.

[stvhay]

This is probably the most controversial pull request, so I've done it based on the assumption the others will be accepted (there were some conflicts)

There might be a cleaner way to do all of this, but it was the best I could come up with. I think the cleaner way might be to figure out how to avoid the from main import stuff that I had to put guards around and creating a wsgi.py entrypoint.