returns shard aware participant ids from discussion view api

fixes CNVS-10549

test plan
- create a discussion topic
- reply to the topic with a user on another shard
- view the discussion as the original topic creator
- ensure that the reply shows the correct author

Change-Id: I5115f6fd3a21f6c8b986e7a9e0c95b5fe4be1587
Reviewed-on: https://gerrit.instructure.com/30080
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Zach Pendleton <zachp@instructure.com>
Product-Review: Zach Pendleton <zachp@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>