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
Technically there is a naming conflict, just one that you may get away with thanks to implementation details. Most users wouldn't understand the difference between a `FileField`, a `ForeignKey` and an `IntegerField` so there are some advantages to "always complain" to enforce a best practice. That said I get your point about warnings being immediately actionable and this being a point release, so let's just remove the warning part (or at least for the time being).