Packaging as reusable Django app

Arklet was made as a standalone service, but it might make sense to package it as a reusable Django app too. We'll be creating some "about" content for Internet Archive. This content doesn't belong in the public repo; it wouldn't be relevant to other groups.

I'm considering creating a separate repo for Internet Archive's deployment which pulls in a package built from this repo as a reusable app. Even if we do that, I think it's useful to keep this repo as something that runs as-is. Developers should have a choice between running this repo as is, or adding the `ark` app to their new or existing Django site.

https://docs.djangoproject.com/en/4.0/intro/reusable-apps/