don't add moodle migration issues for every question

test plan:
* import the moodle package referenced in the ticket
* confirm that there are 2 all-inclusive migration issues
 for the formula questions and multiple dropdown questions
 pointing to the two question banks, rather than many issues
 for each question.

fixes #CNVS-12714

Change-Id: I4de8e3c875e81067997efbb2a3ed8c45a650af8c
Reviewed-on: https://gerrit.instructure.com/34097
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>