migrations: Clean up non one-to-one confirmations/invites.

These models should be one-to-one.  Various bugs in the past have
leaked violations in both directions; we clean them up such that every
PreregistrationUser which is not a multi-use invite has exactly one
Confirmation object associated with it.

Fixes: #22025

zerver/migrations/0516_fix_confirmation_preregistrationusers.py

@@ -0,0 +1,59 @@
+# Generated by Django 5.0.5 on 2024-04-29 19:24
+
+from django.db import migrations
+
+TOO_MANY_CONFIRMATIONS = """
+DELETE FROM confirmation_confirmation
+WHERE  id IN (WITH too_many_confirmations
+                   AS (SELECT object_id,
+                              MIN(confirmation_confirmation.id) AS exclude_id,
+                              COUNT(*)
+                       FROM   confirmation_confirmation
+                       JOIN   zerver_preregistrationuser
+                       ON     content_type_id = (SELECT id
+                                                 FROM   django_content_type
+                                                 WHERE  app_label = 'zerver'
+                                                 AND    model = 'preregistrationuser')
+                       AND    object_id = zerver_preregistrationuser.id
+                       WHERE  referred_by_id IS NOT NULL
+                       GROUP  BY object_id
+                       HAVING COUNT(*) > 1)
+              SELECT id
+               FROM  confirmation_confirmation
+                     JOIN too_many_confirmations
+                       ON too_many_confirmations.object_id = confirmation_confirmation.object_id
+                       AND content_type_id = (SELECT id
+                                              FROM   django_content_type
+                                              WHERE  app_label = 'zerver'
+                                              AND    model = 'preregistrationuser')
+                       AND id != exclude_id
+)
+"""
+
+NO_CONFIRMATIONS = """
+DELETE FROM zerver_preregistrationuser
+ WHERE NOT EXISTS(SELECT 1
+                    FROM confirmation_confirmation
+                   WHERE content_type_id = (SELECT id
+                                            FROM   django_content_type
+                                            WHERE  app_label = 'zerver'
+                                            AND    model = 'preregistrationuser')
+                     AND object_id = zerver_preregistrationuser.id)
+   AND referred_by_id IS NOT NULL
+"""
+
+
+class Migration(migrations.Migration):
+    atomic = False
+
+    dependencies = [
+        (
+            "zerver",
+            "0515_rename_named_group_can_mention_group_namedusergroup_can_mention_group_and_more",
+        ),
+    ]
+
+    operations = [
+        migrations.RunSQL(TOO_MANY_CONFIRMATIONS, elidable=True),
+        migrations.RunSQL(NO_CONFIRMATIONS, elidable=True),
+    ]