Select user(s) to be notified

Added the ability to configure the user reference field from the entity to be moderated in the notification.
You need to add the user reference field to the moderated entity and configure it on the notification. And when a user will create new content, they can choose a user/users to moderate.

Hello @rob-holmes, @jhedstrom

Could you please review this?

Thank you

Just like to add a voice here and say that the patch created by @tyapchyc works as expected.