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/management/commands/validation_command.py

@@ -0,0 +1,11 @@
+from django.core.management.base import NoArgsCommand
+
+
+class InvalidCommand(NoArgsCommand):
+    help = ("Test raising an error if both requires_system_checks "
+            "and requires_model_validation are defined.")
+    requires_system_checks = True
+    requires_model_validation = True
+
+    def handle_noargs(self, **options):
+        pass