re-add reply messaging to email notifications

fixes CNVS-15068

test plan
- cause a conversation message notification email to be sent
- ensure that the text and html versions of the email include text
 saying that you can reply to the email to reply to the
 conversation message
- cause a discussion entry notification email to be sent
- ensure that the html version of the email includes text saying
 that you can reply to the email to reply to the comment (the text
 version should have the text, too, but it was there before this
 commit)

Change-Id: I79c40ecb55bd17eef96931bbb3f255768914fcc7
Reviewed-on: https://gerrit.instructure.com/39925
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>