don't show recent feedback for other students to former admins

test plan:
* create a course
* add two teachers (I'll refer to them as
 "New Teacher" and "Old Teacher")
* also enroll a student

* create an assignment requiring submissions
* as the student, make a submission
* now, as the "New Teacher", comment on the submission
* remove the "Old Teacher" from the course as a teacher,
and instead add them again as a student
* confirm that the "Old Teacher"'s recent feedback does not
show the feedback for the other student

fixes #CNVS-13215

Change-Id: Ie03c9036b65c0de93c869818ebc60b059e9a4260
Reviewed-on: https://gerrit.instructure.com/37465
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>