Run testproj in a Heroku demo app (#38)

* 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

docs/conf.py

@@ -205,7 +205,7 @@ sys.path.insert(0, os.path.abspath('../src'))
 
 # activate the Django testproj to be able to succesfully import drf_yasg
 sys.path.insert(0, os.path.abspath('../testproj'))
-os.putenv('DJANGO_SETTINGS_MODULE', 'testproj.settings')
+os.putenv('DJANGO_SETTINGS_MODULE', 'testproj.settings.local')
 
 from django.conf import settings  # noqa: E402