Add support for more users / authors in notifications

[stevejburge]

One user has requested the ability to notify the author of a post when it expires

[andergmartins]

@stevejburge Maybe this is an opportunity to implement an integration to PublishPress' notifications workflow? We have a good basis for that on PublishPress.

[stevejburge]

Yes, that might be the best option here rather than spending too much time with the basic integration