Show creation date, not edited date, above discussion replies

fixes CNVS-8332

test plan:
  - create a discussion topic
  - reply to said discussion topic
  - a minute later, edit the reply
  - ensure that the time shown above the reply is the time you
    originally created the reply, not the time at which you
    edited it

Change-Id: Ie00e8920c3117fffe219ce941a3565a63a2234b7
Reviewed-on: https://gerrit.instructure.com/40302
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>