Move externalsettings module to external repo and add to boilerplate

Move the externalsettings module from https://gitlab.developers.cam.ac.uk/uis/devops/gsuite/g-suite-authenticator/-/merge_requests/2 into a stand-alone repo and add it to this boilerplate as a dependency. Copy the plumbing from that MR to parse the EXTRA_SETTINGS_URLS environment variable.