These changes:
- Fix the lint and the djmaster jobs
- Fix compatibility with upcoming Django 3.0
- Replace jobs with matrix in .travis.yml
- Add a test job for Python 3.8
- Allow running tests on any branch
* Add Heroku configuration
* Add links in API description
* Read database connection string from DATABASE_URL environment variable
* Restructure settings files for production
* Run server using gunicorn and servce static files with whitenoise
* Install drf-yasg from source instead of pypi in testproj
* Add readme links to demo app