Add sign up and login flows

[zachgoldstein]

Built-in django user model to kick it off? Will this work well enough or do we need to introduce a different system?

[zachgoldstein]

This was a total pain. The built-in django auth and account mgmt stuff is non-trivial and it isn't super clear how to pop it into an application. Had to piece it together from the docs, got stuck. I'm really not happy with how flaky the built-in form functionality is, feels like this makes the auth stuff pretty brittle. Works for now but it's not ideal. Would be interesting to try and move to htmx & shoelace components