Initial implementation of externalsettings module
Provide an initial implementation of this module.
This MR is comprised of two commits.
The first commit adds a cut-down version of our standard boilerplate with everything except testing-related functionality removed. This "stub" project is required to allow use of the Django testing infrastructure.
The second commit adds the extrasettings module from https://gitlab.developers.cam.ac.uk/uis/devops/gsuite/g-suite-authenticator/-/merge_requests/2 and documents its use in the README.