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.
Oh wait--I see what's going on. I think this should be `src/richard/settings_local.py` and it should also tell the user to go look at the docs.