Show full timestamps on conversation messages

Fixes CNVS-13871

Test plan:
  - Create and send a conversation message
  - Locate the message under "Sent" and click on it
  - Ensure that the the full date and time, including the year, are
    displayed in the upper-right corner of the message

Change-Id: I00c3c0df252dc8e32e4aeb2567460c7507750dda
Reviewed-on: https://gerrit.instructure.com/40675
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Janelle Seegmiller <jseegmiller@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>