realm: Add datetime field for scheduled deletion of demo orgs.

This commit is a part of #19523.

zerver/migrations/0342_realm_demo_organization_scheduled_deletion_date.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.2.5 on 2021-08-12 18:41
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("zerver", "0341_usergroup_is_system_group"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="realm",
+            name="demo_organization_scheduled_deletion_date",
+            field=models.DateTimeField(default=None, null=True),
+        ),
+    ]

zerver/models.py

@@ -216,6 +216,9 @@ class Realm(models.Model):
     string_id: str = models.CharField(max_length=MAX_REALM_SUBDOMAIN_LENGTH, unique=True)
 
     date_created: datetime.datetime = models.DateTimeField(default=timezone_now)
+    demo_organization_scheduled_deletion_date: datetime.datetime = models.DateTimeField(
+        default=None, null=True
+    )
     deactivated: bool = models.BooleanField(default=False)
 
     # Redirect URL if the Realm has moved to another server