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
Yes, I agree that it's not a distinction we should expect users to grasp. I'm just trying to weigh the downsides of "users might get confused when one kind of `check` field works but another raises an error" vs the downsides of "compound a back-compat break in 1.7 with another, probably even more common, one in 1.7.1." And it seems to me that the latter is worse.

I think switching to "always error" in 1.8 might make sense.