12795 custom user model

[arthanson]

Fixes: #12795

Did custom user model - dumped and compared SQL schemas and they look correct. Any extra testing suggestions welcome.

Did not do groups for several reasons:

1. All suggestions (SO mostly) suggest just sub-classing Group: https://stackoverflow.com/questions/2181039/how-do-i-extend-the-django-group-model, https://www.reddit.com/r/django/comments/lvakzm/how_should_i_customize_the_group_model/ - Not sure if there is anything in Django that would be an issue with a custom group model (I don't think so off hand) but Django doesn't have any get_group_model like it does for User.
2. There is no BaseGroup or AbstractBaseGroup - although Group model is very simple.
3. There is an old Django ticket (https://code.djangoproject.com/ticket/29748) to allow swapping group model, although nothings been done in 2 years. Bit worried if we do a custom group and Django does theirs it will cause issues (though more minor concern).
4. There is no pressing current need to have a custom Group, then only potential suggestion is for adding a description.
5. Adding a description would not require anything besides subclassing it as it wouldn't be a perf hit, description would'nt be needed in perf critical areas.

Wouldn't be a big thing to convert Groups bug just some potential issues and not a big reason to do it right now IMHO. Also, an option is to potentially do the work for Django and submit a PR - though more work to handle the relation between customizable User and Group models.

[jeremystretch]

Nice work! On first pass this seems to work just fine, however I'm extremely cautious of running across an unforeseen landmine. Will continue to do some further experimentation. We also need to figure out what to do with the NetBoxUser content type.

[jeremystretch]

One other nit: We'll need to ensure that the user and group content types are available in ContentType.objects.public().

[arthanson]

One other nit: We'll need to ensure that the user and group content types are available in ContentType.objects.public().

@jeremystretch updated the registry code and updated the migration to migrate the CustomField from any existing ct pointers to the new one.