Add possibility to read settings from config file

[timobrembeck]

• Read settings from environment variables in settings.py
• Set DEBUG mode to False by default
• Read config file into environment variables in manage.py and wsgi.py
• Document all settings
• Add example config file
• Update README

[julled]

what is part of this config file?

[timobrembeck]

@julled see https://github.com/digitalfabrik/leeway/blob/main/example-config/opendrift-leeway-webgui.ini