Exclude created_by_id from TemplateSchemaNoDetail

We were already excluding `created_by`, but in a different branch we
renamed the property on the schema to `created_by_id`:
https://github.com/alphagov/notifications-api/pull/2873/files#diff-691350bfe8029e08252edaad69e871b9R346-R348

We need to rename it in the exclude list to make sure that this
assertion still passes:
```
assert json_response['data'][0].keys() == {
    'folder',
    'id',
    'name',
    'template_type',
}
```
– 14e7b6b87e/tests/app/template/test_rest.py (L555-L560)

app/schemas.py

@@ -369,7 +369,7 @@ class TemplateSchemaNoDetail(TemplateSchema):
             'archived',
             'content',
             'created_at',
-            'created_by',
+            'created_by_id',
             'hidden',
             'postage',
             'process_type',