Remove auth's custom user test models (their tables aren't created since Django 1.8)

[Simon Charette]

Since the introduction of contrib application migrations in 1.8 (#22170) the ​documented custom user test models table are not created anymore because they are registered to the 'auth' labeled application but are not part of its shipped migrations.

Based on the fact this was not reported before and that the only uses of ExtensionUser I can find is ​either from a vendored version of Django or a copy-pasta of the documentation suggested usage I think we can safely assume it's not commonly used among third-party application who claim to support custom user models. I'll make sure to post the mailing list in order to gather feedback from the community about this point.

Given the models are simply not usable on 1.8 and 1.9 now I'd like to start the deprecation in 1.8 in order order to be able to completely remove them on master but if it's not considered safe enough I came up with a with a workaround that could be backported since there's still no documented way to ship test only models (#7835).

If these models are relied on by the community I would suggest we start deprecating them in 1.10 and adjust the documentation to suggest subclassing the provided abstract models instead.

[Tim Graham]

I've ​written to django-developers with my take on it.

[Tim Graham]

My ​PR for documentation removal of the broken models (to be backported to 1.8).
Simon's ​PR for removing the models from the public API.

[Tim Graham <timograham@…>]

In 1e915044:

Refs #26089 -- Removed obsolete docs about custom user model testing.

[Tim Graham <timograham@…>]

In ea2d9f0:

[1.8.x] Refs #26089 -- Removed obsolete docs about custom user model testing.

Backport of 1e9150443e5696d764ed81c97b53ef0365a5d854 from master

[Tim Graham <timograham@…>]

In 194bf8c:

[1.9.x] Refs #26089 -- Removed obsolete docs about custom user model testing.

Backport of 1e9150443e5696d764ed81c97b53ef0365a5d854 from master

[Tim Graham]

​PR needs a mention in the 1.10 release notes just be on the safe side.

[Simon Charette]

Here's the ​PR with release notes.

[Simon Charette <charette.s@…>]

In 6eb3ce11:

Fixed #26089 -- Removed custom user test models from public API.

Thanks to Tim Graham for the review.