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
And it's a weird warning to be raising. Basically it's a warning that says "your code works fine and will continue to work fine, but it's kind of confusing because it's almost a naming conflict, but not actually one, so we recommend that you change it even though it works."