don't convert relative answer tolerances to floats

test plan:
* create a formula question
* use relative margin of error (e.g. '1%')
* copy the course
* should not have turned the '1%' into a '1'

fixes #CNVS-10580

Change-Id: Ie31b7dafc083b1be704ea21534ce8a2cb0eff3b2
Reviewed-on: https://gerrit.instructure.com/37521
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>