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
I can see the appeal here; however, my gut tells me it's just a little too fiddly to be reliable. From a documentation perspective, "you can't use `check` as a name" is a lot easier to document than "sometimes you can...". I think it's better to just raise an error universally.