Build demo production settings

[justinrporter]

• Remove production.yml in favor of production-demo.yml. A generic production.yml was probably never going to work anyway, and the demo nicely serve as a demo for users (since it's hosted live) and a demo for implementers (since we commit the settings to our repo).
• Move POSTGRES_HOST=kc-med-oslerdb.kc.umkc.edu from ./.envs/.production/.postgres (where it obviously only applies to UMKC) to production-umkc.yml
• Remove traces of Traefik
• Create .envs/.secrets/.gitignore that ignores everthing in that directory. Can be used as a repository for secrets.
• Create .compose/production/nginx/certs/.gitignore to automatically git-ignore any SSL certs placed there.
• Create docs/running-in-production.rst to describe the process of setting up LLEMR in production.
• Update postgres from 11.3 to 11.12