add reply messaging to email notifications

fixes CNVS-5793

test plan:
  * set a user's notification settings to ASAP;
  * as another user, do the following and verify that
    the email received contains messaging informing the
    user that they can reply directly to the email:
  * send a conversation message;
  * comment on a discussion topic that the first user is
    subscribed to;
  * comment on one of the user's assignment submissions.

Change-Id: Iffe0763aad26579e5b5955f4f31e5e5242c14355
Reviewed-on: https://gerrit.instructure.com/29504
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Braden Anderson <banderson@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Product-Review: Zach Pendleton <zachp@instructure.com>