fix discussion_topic importing into groups

test plan:
* create a course with a discussion topic
* export the course
* create a new course
* add a user group in the new course
* import the package into the course
* should not fail or import as a group topic

fixes #CNVS-11377

Change-Id: Ied4c80ed48d6495bdee60f79948445141997d5e8
Reviewed-on: https://gerrit.instructure.com/31087
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>