*8895* Include PKP PLN plugin in OJS stable

Giving a duplicate notification to every journal manager of a journal is probably going to cause confusion, but since we can't guarantee that a single user (e.g. the primary contact or tech contact) is also going to have a user ID, I can't think of a clear improvement. Perhaps check first to see if there's a single preferable user based e.g. on the tech and primary contacts and fall back on this if needed?