Update reference to User model in Organization and UserMeta models to `settings.AUTH_USER_MODEL`

[princess-fixturfab]

In django 3.2, Referencing the User Model:

If you reference [User](https://docs.djangoproject.com/en/3.2/ref/contrib/auth/#django.contrib.auth.models.User) 
directly (for example, by referring to it in a foreign key), your code will not work in projects where the
[AUTH_USER_MODEL](https://docs.djangoproject.com/en/3.2/ref/settings/#std-setting-AUTH_USER_MODEL) 
setting has been changed to a different user model.

This includes the owner field in Organization model and user field in UserMeta model.