refactor Assignment#anonymous_student_identities
refactor Assignment#anonymous_student_identities

flag=none

Test Plan:
- specs pass

Change-Id: Ia87499b6160db8b07ae5507cf456b7ef800064e0
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/289197
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Eduardo Escobar <7e82d09bba767b0dd1222915e3d92141089e94a8@instructure.com>
Reviewed-by: Kai Bjorkman <4693b3d9e71784d629732770ab5e83ed8fbdb83e@instructure.com>
QA-Review: Spencer Olson <7cc0890d6f1f70810a023e7173eca5724d6c8e49@instructure.com>
Product-Review: Spencer Olson <7cc0890d6f1f70810a023e7173eca5724d6c8e49@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms