Settings redux

This is a redux of willkg/richard#242
* The movement of files is kept mostly in place (siloing files under /src)
* The settings config went through another iteration to django-configurations
* Documentation updates galore

The change to django-configurations means mod_wsgi won't work anymore. If that's used in prod, then the changeover to something like nginx / gunicorn isn't too hard but it going to require a good round of testing in dev.
I was thinking we could "cheat" here and do something like:

```
if 'test' in sys.argv:
    os.environ.setdefault('DJANGO_CONFIGURATION', 'Testing')
```

Does that create unfortunate edge cases?