extend notification options to include options to send to *all* post instructors

[thomasplevy]

Notifications for courses & membership that send to course author will currently only send the notifications to the primary instructor (the author on the wp_posts table).

Need an additional option to send to secondary instructors (possibly by role) as defaults on the notification settings

realistically this may need to be added as settings on a course level:

eg admins can setup global notifications and instructors should be able to override that on a course by course basis per their preferences. Especially since instructors (and assistants) cannot access the notifications settings