fix question bank de-duping on import

test plan:
* create a course with two question banks with
 the same name, each containing at least one question
* create a quiz with quiz groups that pick from the
 question banks
* copy the course
* should retain two separate question banks, and retain
the quiz groups
* the questions (and number of them) for each bank should
remain the same as the original course

closes #CNVS-14780

Change-Id: I8ed819220021cf965e3ca90206c7096e5bde717c
Reviewed-on: https://gerrit.instructure.com/42392
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>