fix observee student enrollment constraint violation

test plan:
 1. Set up a user account (A) with a login.
 2. As another user (B), observe the first user via
    /profile/observees
 3. Enroll user A as a student in a course. User B is
    automatically enrolled as an observer.
 4. Remove the observer from the course.
 5. Attempt to enroll user A in a custom student role in the
    course (in addition to the normal Student enrollment
    created in step 3).
    -> This should succeed without an AJAX error.
    -> The observer should be re-enrolled in the course.
 6. Repeat steps 3 and 5 in a new course (don't delete the
    observer enrollment).
    -> This should succeed without an AJAX error.

fixes CNVS-16521

Change-Id: I0d2ea2d4f9b3dc2e32c33052c541df356366d286
Reviewed-on: https://gerrit.instructure.com/43436
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>