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
Quoting Russell on IRC: 

    freakboy3742: There's also an argument to be made that this actually shouldn't be a check at all, but a fundamental system error.
    freakboy3742: There are some other protected names - like save - and Django Just Wont Work if you use those names.

I agree with him, if you silence this warning, you also silence every other warnings for this model and its fields which could be a serious foot-gun. Could you cc all involved in the discussion?