Bug 31627: Add ability to embed the letter id in outgoing email notices

Bug 31627: Add ability to embed the letter id in outgoing email notices

It can be useful to know exactly what template was used to generate a notice. To further this end, it would be useful be able to send the letter id as part of any emails sent out if there is a letter template associated with the message.

Test Plan:
1) Apply this patch
2) Restart all the things!
3) Generate an email notice like a checkout notice
4) Inspect the email headers, note the new X-Koha-Template-Id and
   X-Koha-Message-Id headers!

Signed-off-by: Sam Lau <samalau@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>