conversations: mark as read when replying by email

fixes CNVS-13245

test plan:
  * set up reply by email
  * start a conversation with a user receiving conversation emails
  * reply by email as the recipient
  * open conversations as the recipient
  * verify that the conversation is marked as read

Change-Id: Ic52639abea1cc8d9e0214453e9a1dc1c1b4c6906
Reviewed-on: https://gerrit.instructure.com/35319
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ethan Vizitei <evizitei@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>