don't reuse question banks on import unless specified

test plan:
* import a qti quiz package into a course
* re-import it, making sure to *not* check the
 "Overwrite assessment content with matching IDs" checkbox

* should not put all of the questions into the same question bank

* repeating the previous while checking the box should behave as
 it currently does

closes #CNVS-14669

Change-Id: Ibfed20a191986fa29e52d7a08313b879c917d5a6
Reviewed-on: https://gerrit.instructure.com/41735
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>