Security/convenience improvement for settings.py
This change allows us to set a local_settings.py file on a deployed
instance, containing site-specific settings like the LOGIN_URL_REDIRECT
parameter or db config info. Particularly, this will allow us to keep
operational details of particular deploy stacks (eg, db passwords and
site secret keys) out of the repo, while allowing non-confidential
settings updates to hit the repo - and there is no need to preserve the
old settings.py file when doing a build.