Implement the structured groups backend

This implements a structured user group model, propagating its changes
to vanilla Django groups. It features cycle detection at save-time,
although this cycle detection can only be seen as a fallback and forms
should check the acyclicity of their changes before committing using the
provided class method.

wiki_groups/migrations/0001_initial.py

@@ -0,0 +1,27 @@
+# Generated by Django 2.2.8 on 2019-12-16 00:01
+
+from django.conf import settings
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    initial = True
+
+    dependencies = [
+        ('auth', '0011_update_proxy_permissions'),
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='WikiGroup',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('django_group', models.OneToOneField(on_delete=django.db.models.deletion.CASCADE, to='auth.Group')),
+                ('includes_groups', models.ManyToManyField(blank=True, related_name='included_in_groups', to='wiki_groups.WikiGroup')),
+                ('users', models.ManyToManyField(blank=True, to=settings.AUTH_USER_MODEL)),
+            ],
+        ),
+    ]