Access to old state to NewsPublisher and PostProcessor

Current implementation of `NewsPublisher` and `PostProcessor` has a signature of `(Action, Effect, State) -> T?`, which can be somehow limiting, when you want to diff the previous and current state.
The common suggestion to update the signature to include the previous state, e.g. `(State, Action, Effect, State) -> T?`, which seems to be the best idea for now.

Locate the NewsPublisher and PostProcessor definitions and their usages, then trace how the current state reaches their callbacks. Confirm the API impact of adding the previous state to the signature; done means both callbacks can receive previous and current state and the affected project checks pass.

Written by the indexing model from the issue text.