Fixed #16905 -- Added extensible checks (nee validation) framework

This is the result of Christopher Medrela's 2013 Summer of Code project.

Thanks also to Preston Holmes, Tim Graham, Anssi Kääriäinen, Florian
Apolloner, and Alex Gaynor for review notes along the way.

Also: Fixes #8579, fixes #3055, fixes #19844.

tests/validation/test_error_messages.py

@@ -5,7 +5,6 @@ from unittest import TestCase
 
 from django.core.exceptions import ValidationError
 from django.db import models
-from django.utils import six
 
 
 class ValidationMessagesTest(TestCase):
@@ -19,10 +18,6 @@ class ValidationMessagesTest(TestCase):
         f = models.AutoField(primary_key=True)
         self._test_validation_messages(f, 'fõo',
             ["'fõo' value must be an integer."])
-        # primary_key must be True. Refs #12467.
-        with six.assertRaisesRegex(self, AssertionError,
-                "AutoFields must have primary_key=True."):
-            models.AutoField(primary_key=False)
 
     def test_integer_field_raises_error_message(self):
         f = models.IntegerField()