Add custom user model

[rhysyngsun]

NOTE: #2013 should be reviewed first, as changes from it are included here.

What are the relevant tickets?

Part of https://github.com/mitodl/hq/issues/6679

Description (What does it do?)

This adds a custom user model and points the AUTH_USER_MODEL to it.
It updates the implementation of the 0001_initial.py model by having deleted this file and then recreating it after updating AUTH_USER_MODEL.

How can this be tested?

• Ensure you have run the migrations from this PR before proceeding so that 0001_initial.py gets recorded as having been run. If it's been merged, you can just go ahead and run the migration off main.
• Checkout this branch and test that ./manage.py showmigrations still lists 0001_initial.py under the users app as having been applied.
• Run ./manage.py makemigrations --dry and verify there are no changes.
• Start the app, verify the migrations run, and the app is functional.

When you are done with testing, be sure to run ./manage.py migrate users 0001 to revert not break things on other branches.

[shanbady]

I'm getting this error when trying to run show migrations

Traceback (most recent call last):
  File "/opt/venv/lib/python3.12/site-packages/django/apps/config.py", line 235, in get_model
    return self.models[model_name.lower()]
           ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^
KeyError: 'user'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/venv/lib/python3.12/site-packages/django/contrib/auth/__init__.py", line 170, in get_user_model
    return django_apps.get_model(settings.AUTH_USER_MODEL, require_ready=False)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/venv/lib/python3.12/site-packages/django/apps/registry.py", line 213, in get_model
    return app_config.get_model(model_name, require_ready=require_ready)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/venv/lib/python3.12/site-packages/django/apps/config.py", line 237, in get_model
    raise LookupError(
LookupError: App 'users' doesn't have a 'User' model.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/src/./manage.py", line 29, in <module>
    execute_from_command_line(sys.argv)
  File "/opt/venv/lib/python3.12/site-packages/django/core/management/__init__.py", line 442, in execute_from_command_line
    utility.execute()
  File "/opt/venv/lib/python3.12/site-packages/django/core/management/__init__.py", line 416, in execute
    django.setup()
  File "/opt/venv/lib/python3.12/site-packages/django/__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/opt/venv/lib/python3.12/site-packages/django/apps/registry.py", line 116, in populate
    app_config.import_models()
  File "/opt/venv/lib/python3.12/site-packages/django/apps/config.py", line 269, in import_models
    self.models_module = import_module(models_module_name)
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/importlib/__init__.py", line 90, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen importlib._bootstrap>", line 1387, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1360, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1331, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 935, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 995, in exec_module
  File "<frozen importlib._bootstrap>", line 488, in _call_with_frames_removed
  File "/src/profiles/models.py", line 49, in <module>
    User = get_user_model()
           ^^^^^^^^^^^^^^^^
  File "/opt/venv/lib/python3.12/site-packages/django/contrib/auth/__init__.py", line 176, in get_user_model
    raise ImproperlyConfigured(
django.core.exceptions.ImproperlyConfigured: AUTH_USER_MODEL refers to model 'users.User' that has not been installed

[shanbady]

LGTM