LP1593834: Add Date Header to A/T Email Examples

LP1593834: Add Date Header to A/T Email Examples

RFCs 822 and 2822 specify that a Date: header has to be sent
indicating when an email was sent. Without this, clients that
default to displaying the Sent Date may end up using a "NULL"
date, or 1970-01-01 00:00:00 UTC, which is not ideal. This
adds the header to the example A/T templates for email and
SMS messages, and includes instructions on correcting
existing tempalates.

Signed-off-by: Jason Boyer <jboyer@library.in.gov>
Signed-off-by: Mike Rylander <mrylander@gmail.com>