Use get_user_model for reference to User

[arthanson]

Proposed Changes

There is a large number of places in the code that import and use the User model directly:

from django.contrib.auth.models import User

These should all be replaced with calls to get_user_model to facilitate the potential change to a custom User model.

Justification

See https://docs.djangoproject.com/en/4.2/topics/auth/customizing/#referencing-the-user-model - this is a requirement if we are going to potentially use a custom user model.