Upgrade Django

[bensteinberg]

This is a major version upgrade of Django. This is a first cut; tests pass locally, but I haven't examined the running application. I'm creating the PR in case anyone wants to take a look.

https://docs.djangoproject.com/en/4.2/releases/4.2.16/

https://docs.djangoproject.com/en/4.2/releases/4.2/

https://docs.djangoproject.com/en/4.2/releases/4.2/#backwards-incompatible-4-2

[rebeccacremona]

I see no important warnings/deprecations when running the tests locally; I can give the application a bit of a spin.

[rebeccacremona]

Everything LGTM: I logged in locally and clicked around in the admin, and also ran it with my local Perma, and confirmed that if I initiated a purchase request, I was successfully forwarded to Cybersource's testing tier.