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/admin_scripts/app_raising_messages/models.py

@@ -0,0 +1,27 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.core import checks
+from django.db import models
+
+
+class ModelRaisingMessages(models.Model):
+    @classmethod
+    def check(self, **kwargs):
+        return [
+            checks.Warning(
+                'First warning',
+                hint='Hint',
+                obj='obj'
+            ),
+            checks.Warning(
+                'Second warning',
+                hint=None,
+                obj='a'
+            ),
+            checks.Error(
+                'An error',
+                hint='Error hint',
+                obj=None,
+            )
+        ]