Feedback mod: Email notifications sent to expired teaching staff

When the notification of submissions is set to "yes", student submissions are sent to all teaching staff regardless of whether they are active or not.

Testing instructions:

1. Create a feedback activity and set "send notifications" to yes
2. enrol a teacher and set their enrolment expiry to be in the past so that they are showing as inactive
3. log in as a student and complete the feedback
4. Currently the expired staff member will receive the email.