migration: Add migration to remove default status of private streams.

This commit adds migration which removes default status of exisitng
default private streams, i.e. private stream exists but they are no
longer default.

zerver/migrations/0304_remove_default_status_of_default_private_streams.py

@@ -0,0 +1,22 @@
+# Generated by Django 2.2.14 on 2020-08-10 20:21
+
+from django.db import migrations
+from django.db.backends.postgresql.schema import DatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+
+
+def remove_default_status_of_default_private_streams(apps: StateApps, schema_editor: DatabaseSchemaEditor) -> None:
+    DefaultStream = apps.get_model('zerver', 'DefaultStream')
+    DefaultStream.objects.filter(stream__invite_only=True).delete()
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0303_realm_wildcard_mention_policy'),
+    ]
+
+    operations = [
+        migrations.RunPython(remove_default_status_of_default_private_streams,
+                             reverse_code=migrations.RunPython.noop,
+                             elidable=True),
+    ]