[1.9.x] Refs #19353 -- Added tests for using custom user models with built-in auth forms.

Also updated topics/auth/customizing.txt to reflect that subclasses of
UserCreationForm and UserChangeForm can be used with custom user models.

Thanks Baptiste Mispelon for the initial documentation.

Backport of f0425c7260 from master

tests/auth_tests/test_forms.py

@@ -10,6 +10,7 @@ from django.contrib.auth.forms import (
     SetPasswordForm, UserChangeForm, UserCreationForm,
 )
 from django.contrib.auth.models import User
+from django.contrib.auth.tests.custom_user import ExtensionUser
 from django.contrib.sites.models import Site
 from django.core import mail
 from django.core.mail import EmailMultiAlternatives
@@ -153,6 +154,21 @@ class UserCreationFormTest(TestDataMixin, TestCase):
             form['password2'].errors
         )
 
+    def test_custom_form(self):
+        class CustomUserCreationForm(UserCreationForm):
+            class Meta(UserCreationForm.Meta):
+                model = ExtensionUser
+                fields = UserCreationForm.Meta.fields + ('date_of_birth',)
+
+        data = {
+            'username': 'testclient',
+            'password1': 'testclient',
+            'password2': 'testclient',
+            'date_of_birth': '1988-02-24',
+        }
+        form = CustomUserCreationForm(data)
+        self.assertTrue(form.is_valid())
+
 
 @override_settings(USE_TZ=False, PASSWORD_HASHERS=['django.contrib.auth.hashers.SHA1PasswordHasher'])
 class AuthenticationFormTest(TestDataMixin, TestCase):
@@ -441,6 +457,24 @@ class UserChangeFormTest(TestDataMixin, TestCase):
         # value to render correctly
         self.assertEqual(form.initial['password'], form['password'].value())
 
+    def test_custom_form(self):
+        class CustomUserChangeForm(UserChangeForm):
+            class Meta(UserChangeForm.Meta):
+                model = ExtensionUser
+                fields = ('username', 'password', 'date_of_birth',)
+
+        user = User.objects.get(username='testclient')
+        data = {
+            'username': 'testclient',
+            'password': 'testclient',
+            'date_of_birth': '1998-02-24',
+        }
+        form = CustomUserChangeForm(data, instance=user)
+        self.assertTrue(form.is_valid())
+        form.save()
+        self.assertEqual(form.cleaned_data['username'], 'testclient')
+        self.assertEqual(form.cleaned_data['date_of_birth'], datetime.date(1998, 2, 24))
+
 
 @override_settings(
     PASSWORD_HASHERS=['django.contrib.auth.hashers.SHA1PasswordHasher'],