keep assessment question and quiz question migration ids separate

fixes a difficult-to-reproduce problem with export packages
that causes quiz questions to be swapped with other quiz questions
that happen to have the same db id number as related assessment
questions

fixes #CNVS-14365

Change-Id: I0d7ccfd6bf4f3ef10169443ca4e994dcc935f053
Reviewed-on: https://gerrit.instructure.com/38043
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>