Add the filename to the subject line of Protocol Action

and Document Action messages.  This fixes ietf-tools#507.
 - Legacy-Id: 2620

Author: fenner

fenner/ietf/templates/idrfc/approval_mail.txt

@@ -1,7 +1,7 @@
 From: The IESG <iesg-secretary@ietf.org>
 To: IETF-Announce <ietf-announce@ietf.org>{% if cc %}
 Cc: {{ cc|safe }}{% endif %}
-Subject: {{ action_type }} Action: '{{ doc.title }}' to {{ status }}
+Subject: {{ action_type }} Action: '{{ doc.title }}' {{ doc.file_tag|safe }} to {{ status }}
 
 {% filter wordwrap:73 %}The IESG has approved the following document{{ docs|pluralize }}:{% for d in docs %}
 - '{{ d.filled_title }}'