When we nest the `TemplateSchema` as a field on the
`NotificationWithTemplateSchema`, we want to include the
`is_precompiled_letter` field. However, we don't want the
`is_precompiled_letter` in any of the other places that we use the
`TemplateSchema`.
The way we had the code was giving errors in version 3 of Marshmallow
since `is_precompiled_letter` was not defined on the `TemplateSchema`.
Instead of writing complicated logic around when the field should be
included or excluded, this adds a new schema which we can use in the one
place where we do want to include `is_precompiled_letter`.
57788e5da1