conversations: only mark conversations unread for participants

fixes CNVS-11312

test plan:
  * start a conversation from user A to B and C
  * as user B, mark the conversation as read
  * as user A, send a reply only to user C
  * as user B, verify that the conversation is still read

Change-Id: I7051eb39acd711ad10344e9e17700a0987a72b22
Reviewed-on: https://gerrit.instructure.com/36172
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Drew Bowman <dbowman@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Drew Bowman <dbowman@instructure.com>