fix observer linking after sis import

test plan:
* create a student
* as a user with a valid login,
observe the student permanently using the
observees UI ("/profile/observees")
* enroll the student in a course through
an sis import
* should create an observer enrollment
for the observer as expected

closes #CNVS-14943

Change-Id: I087190b3672b51d2b2f91551ea3a0a424ab89091
Reviewed-on: https://gerrit.instructure.com/42205
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>