use user id instead of user as key for @account_users_cache

Change-Id: I9cecab385e101a1fc035455ff98d7e30defc27be
Reviewed-on: https://gerrit.instructure.com/29463
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>