conversations: fix last_authored_message_at

fixes CNVS-13598

test plan:
  * open your browser's network log
  * reply to a conversation
  * verify that in the response, last_authored_message_at
      matches the current time

Change-Id: I22db516bbe61fd341329fb4f6d3dd8c556edf0b2
Reviewed-on: https://gerrit.instructure.com/36326
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Drew Bowman <dbowman@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Braden Anderson <braden@instructure.com>