Make sure mandatory settings are set in the module

[fricklerhandwerk]

Currently we hard-code some settings in the Django app (such as the GitHub org and repo). When merely testing and not using the staging configuration as a template, this will crash with an inscrutable error. It would be better if we could notify the user that the settings must be set in the module, and give a hint on finding out how/where to do that.

[alejandrosame]

The inscrutable errors are due to the replies given by the GitHub API, which won't be fixed unless they are rewrapped.

However, what this is hinting at is that settings.py shouldn't have values for 3rd party integrations and we should move them to an existence check.

[fricklerhandwerk]

Also, as opposed to dumb key-values in Python, in the module we can extensively document what we need to have set there and check types.