validate presence of user and submission in peer reviews
validate presence of user and submission in peer reviews

closes EVAL-2609
flag=assignments_2_student
flag=peer_reviews_for_a2

test plan:
-test passes

Change-Id: I457444ccdb6cd093b079b2c3c8edcedfe17f7ce7
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/300276
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Spencer Olson <7cc0890d6f1f70810a023e7173eca5724d6c8e49@instructure.com>
Reviewed-by: Kai Bjorkman <4693b3d9e71784d629732770ab5e83ed8fbdb83e@instructure.com>
QA-Review: Kai Bjorkman <4693b3d9e71784d629732770ab5e83ed8fbdb83e@instructure.com>
Product-Review: Deborah Kwak <5498c4751d2b9365fe654627c4991c217ce972a8@instructure.com>

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