confirmation: Add an index on content_type, object_id.

This makes lookups of `.confirmation`, or joins through
`confirmation__` as we do in the `/json/invites`, actually fast.  See
https://code.djangoproject.com/ticket/23435

confirmation/migrations/0014_confirmation_confirmatio_content_80155a_idx.py

@@ -0,0 +1,21 @@
+from django.contrib.postgres.operations import AddIndexConcurrently
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    atomic = False
+
+    dependencies = [
+        ("confirmation", "0013_alter_realmcreationkey_id"),
+        ("contenttypes", "0002_remove_content_type_name"),
+        ("zerver", "0552_remove_realm_private_message_policy"),
+    ]
+
+    operations = [
+        AddIndexConcurrently(
+            model_name="confirmation",
+            index=models.Index(
+                fields=["content_type", "object_id"], name="confirmatio_content_80155a_idx"
+            ),
+        ),
+    ]

confirmation/models.py

@@ -231,6 +231,9 @@ class Confirmation(models.Model):
 
     class Meta:
         unique_together = ("type", "confirmation_key")
+        indexes = [
+            models.Index(fields=["content_type", "object_id"]),
+        ]
 
     @override
     def __str__(self) -> str: