stop stream items from unlinking discussion subentries

fixes CNVS-14383

test plan
- create a discussion with "post before seeing replies" checked
- create a reply to the discussion
- as a user in the course who has not replied to the topic,
 fetch the /api/v1/courses/:course_id/activity_stream/summary
- as the topic creator, edit the topic message (to queue a job
 to recreate the materialized view)
- ensure that the reply is still present (the materialized view
 must be regenerated before the bug would remove the entry)
- toggle the unread state on the entry, ensuring that no errors
 appear in the debug console

Change-Id: Iaf8d15195c5c6fef8f1b22bf7dfc9110b05394f2
Reviewed-on: https://gerrit.instructure.com/38564
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>