migrations: Disable atomic for delivery_email migration.

I'm not sure theoretically why this should be required only for some
installations, but these articles all suggest the root problem is
doing these two migrations together atomically (creating the field and
setting a value for it), so the right answer is to declare the
migration as not atomic:

https://stackoverflow.com/questions/12838111/django-db-migrations-cannot-alter-table-because-it-has-pending-trigger-events
https://confluence.atlassian.com/confkb/upgrade-failed-with-the-error-message-error-cannot-alter-table-content-because-it-has-pending-trigger-events-747606853.html

zerver/migrations/0174_userprofile_delivery_email.py

@@ -15,6 +15,7 @@ def copy_email_field(apps: StateApps, schema_editor: DatabaseSchemaEditor) -> No
 
 
 class Migration(migrations.Migration):
+    atomic = False
 
     dependencies = [
         ('zerver', '0173_support_seat_based_plans'),