*8895* Include PKP PLN plugin in OJS stable

(My inclination here comes from C-land: defines should be brainless and do almost nothing -- they're handled by the pre-processor in C -- and anything that's actually active should happen in functions/classes/etc. I think it's a good division. Instantiating objects for no purpose beyond figuring out class names may have strange side-effects e.g. if other plugins use hooks on them or their DAOs.)