update previewUrl for group discussion topics

fixes EVAL-4625
flag=none

test plan:
- create a group discussion and add some replies
- go to the group discussion in speed grader 2
- verify that you can see the replies

Change-Id: I0abc5e321d7864a4f86ef3f0f6ad45853e363f8b
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/370130


Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Cameron Ray <cameron.ray@instructure.com>
QA-Review: Derek Williams <derek.williams@instructure.com>
Product-Review: Cameron Ray <cameron.ray@instructure.com>