Simplify configuration flow

One source of unexpected bugs in testing was unpredictable settings files chosen (specifically the special casing for `get_local_test_settings_file` in `manage.py`. One way I have dealt with this in other django programs was to use other packages to organize settings in a more extensible fashion.

Some options (from least to most involved):
* [django-environ](https://pypi.org/project/django-environ/) - Pull configuration from environment variables. This would make testing relate with different DBs etc... a lot simpler, and make it much easier to run from a container without needing to override settings files. This could add the option to distribute a Relate docker image that you could run directly without needing to setup your own python environment or webserver
* [django-split-settings](https://pypi.org/project/django-split-settings/) - Split settings classes up into different files. This would allow pulling large configuration chunks like saml into their own files to make it easier for a new user to setup their server with customization without worrying about parts that don't apply to them.
* [django-customizations](https://pypi.org/project/django-configurations/) - Use class based settings options. A user's configuration could inherit from the base settings, test settings could inherit from a shared testing class, etc...

Start with manage.py and its special-cased get_local_test_settings_file path to understand how settings are selected during tests. Compare that flow with the django-environ, django-split-settings, and django-configurations approaches listed in the issue, then establish which approach is wanted. Done should mean predictable test configuration and a clearly defined, easier-to-customize deployment setup.

Written by the indexing model from the issue text.