Port externalsettings infrastructure

This is a direct port of https://gitlab.developers.cam.ac.uk/uis/devops/gsuite/g-suite-authenticator/-/merge_requests/2, review commits and all. It was mostly done via git cherry-pick with some fixups around the edges.

Take the opportunity to bump the Django version to 2.2 and fix a couple of missing features to do with speeding up tox tests and, most crucially, support loading Lookup credentials from external settings to allow for Cloud hosting.

Closes #7 (closed)