conversations: always give copies of messages to authors

fixes CNVS-11517

test plan:
  * create a conversation with a course context in new conversations
  * reply to the conversation by email
  * verify that the new message is visible both to the email author
      and the author of the previous message

Change-Id: I78496d31fb82b773007c22d4478d62061cb9efef
Reviewed-on: https://gerrit.instructure.com/31101
Reviewed-by: Jon Willesen <jonw@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>