Make the dummy Admin model a managed model

Fixes #6810. As per https://code.djangoproject.com/ticket/13816 , unmanaged models are included in dumpdata by design and so unmanaged models without a database table are not valid.

CHANGELOG.txt

@@ -16,6 +16,7 @@ Changelog
 
  * Fix: Ensure aliases are published when the source page is published (Karl Hobley)
  * Fix: Prevent error when saving embeds that do not include a thumbnail URL (Cynthia Kiser)
+ * Fix: Prevent failure when running `manage.py dumpdata` with no arguments (Matt Westcott)
 
 
 2.12 (02.02.2021)

docs/releases/2.12.1.rst

@@ -15,3 +15,4 @@ Bug fixes
 
  * Ensure aliases are published when the source page is published (Karl Hobley)
  * Prevent error when saving embeds that do not include a thumbnail URL (Cynthia Kiser)
+ * Prevent failure when running `manage.py dumpdata` with no arguments (Matt Westcott)

wagtail/admin/migrations/0003_admin_managed.py

@@ -0,0 +1,24 @@
+# Generated by Django 3.1.5 on 2021-02-15 15:32
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('wagtailadmin', '0002_admin'),
+    ]
+
+    operations = [
+        migrations.DeleteModel(name='Admin'),
+        migrations.CreateModel(
+            name='Admin',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+            ],
+            options={
+                'permissions': [('access_admin', 'Can access Wagtail admin')],
+                'default_permissions': [],
+            },
+        ),
+    ]

wagtail/admin/models.py

@@ -13,10 +13,9 @@ from wagtail.core.models import Page
 
 # A dummy model that exists purely to attach the access_admin permission type to, so that it
 # doesn't get identified as a stale content type and removed by the remove_stale_contenttypes
-# management command. managed = False ensures that this doesn't create a database table.
+# management command.
 class Admin(Model):
     class Meta:
-        managed = False
         default_permissions = []  # don't create the default add / change / delete / view perms
         permissions = [
             ('access_admin', "Can access Wagtail admin"),