Fixed #23615 -- Validate that a Model instance's "check" attribute is a ...

...method.

The "check" name is a reserved word used by Django's Check framework,
and cannot be redefined as something else other than a method, or the Check
framework will raise an exception.

This change amends the django.core.checks.model_check.check_all_models()
function, so that it verifies that a model instance's attribute "check"
is actually a method, before calling it and collecting the error
messages.

A test is also provided, and this specific violation is documented and assigned
the id "fields.E008".

This fixes https://code.djangoproject.com/ticket/23615
This does not appear to be possible, due to the way `django.core.checks.models_checks.check_all_models` is implemented. In particular, see: https://github.com/django/django/blob/master/django/core/checks/model_checks.py#L15

From my undertstanding, check_all_models() queries the default app registry directly. Any custom registry will not be taken into consideration when performing the check.

Do you have a suggestion on how to make it work with a custom app registry?