settings: Split 'settings.py' into multiple files

settings: Split 'settings.py' into multiple files

This will make testing/developing a little easier and prevent people
deploying insecure instances.

This also adds references to Django documentation in said files (for
Django novices) and restructures the files to group them together
logically.

This allows us to remove the settings file used for tests.

Signed-off-by: Stephen Finucane <stephenfinucane@hotmail.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>