go back to random numeric identifiers for answers

test plan:
 - import a quiz containing true/false or multiple-choice questions
 - take the quiz
 - the answers you provided should be recorded and scored properly

fixes CNVS-13623

Change-Id: I761f89ea9fca646ed83523cac48c160d957cc14d
Reviewed-on: https://gerrit.instructure.com/36431
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Clare Strong <clare@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>