account setting for author email in notifications

fixes CNVS-15348

test plan
- send a conversation message
- ensure that the notification does not include the author's email
 address
- in the account's settings page, check "Show the email address of
 sender for user interaction Notifications"
- send another conversation message
- ensure that the notification includes the author's email address

Change-Id: I2816d82fb2f6413d1c2c972296a758b571fb03de
Reviewed-on: https://gerrit.instructure.com/40766
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Janelle Seegmiller <jseegmiller@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>