fix discussion importing with attachments

also tweak a couple random little things because why not

test plan:
* create a course with a file
* import the package referenced in the ticket
* should not attach the file to every imported
 discussion topic

closes #CNVS-14520

Change-Id: Icf30b0a62d0d8f8ab6ced84990def1525ff061a4
Reviewed-on: https://gerrit.instructure.com/41760
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>