Change email message on_delete to not block organization delete (#2060)

Change email message on delete so that email messages do not need to be
deleted before deleting an organization/user.

engine/apps/email/migrations/0002_alter_emailmessage_receiver.py

@@ -0,0 +1,20 @@
+# Generated by Django 3.2.19 on 2023-05-30 16:45
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('user_management', '0011_auto_20230411_1358'),
+        ('email', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='emailmessage',
+            name='receiver',
+            field=models.ForeignKey(default=None, null=True, on_delete=django.db.models.deletion.CASCADE, to='user_management.user'),
+        ),
+    ]

engine/apps/email/models.py

@@ -16,5 +16,5 @@ class EmailMessage(models.Model):
         "base.UserNotificationPolicy", on_delete=models.SET_NULL, null=True, default=None
     )
 
-    receiver = models.ForeignKey("user_management.User", on_delete=models.PROTECT, null=True, default=None)
+    receiver = models.ForeignKey("user_management.User", on_delete=models.CASCADE, null=True, default=None)
     created_at = models.DateTimeField(auto_now_add=True)