conversations: update unread count only for recipients

fixes CNVS-13639

test plan:
  * as user A, send a message to B and C
  * as user B, mark the conversation as read
  * as user C, reply privately to A
  * as user B, verify that your unread message count does not change

Change-Id: I9fbf9b7bfea507d51a95231568ec33211ddd14da
Reviewed-on: https://gerrit.instructure.com/36519
Reviewed-by: Drew Bowman <dbowman@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Braden Anderson <braden@instructure.com>
Tested-by: Bryan Madsen <bryan@instructure.com>