Mail notifications have a Reply-To to course teachers

Closes #38.

owl.py

@@ -1682,14 +1682,24 @@ def send_notify_post(post):
         """, (topic.cid, post.target_uid, post.target_uid, topic.tid))
     dests = db.fetchall()
 
+    db_query("""
+            SELECT u.email, u.first_name, u.last_name
+            FROM owl_enroll e
+            JOIN owl_users u ON u.uid = e.uid
+            WHERE e.cid=%s
+              AND e.is_teacher
+              AND u.email IS NOT NULL
+        """, (topic.cid,))
+    reply_to = db.fetchall()
+
     for dest in dests:
         if dest.uid != author.uid or dest.notify_self:
-            send_notify_to_dest(post, topic, course, author, target, dest)
+            send_notify_to_dest(post, topic, course, author, target, dest, reply_to)
         else:
             app.logger.info(f"E-mail: not sending self-notify to uid={dest.uid}")
 
 
-def send_notify_to_dest(post, topic, course, author, target, dest):
+def send_notify_to_dest(post, topic, course, author, target, dest, reply_to):
     if 'MAIL_FROM' not in app.config:
         app.logger.info('Not sending: MAIL_FROM not defined')
         return
@@ -1709,6 +1719,9 @@ def send_notify_to_dest(post, topic, course, author, target, dest):
         # Everybody gets his own view of the thread, so the target is the observer's uid
         msg['References'] = '<' + app.config['MAIL_REFERENCES'].format(tid=post.tid, target=dest.uid) + '>'
 
+    if reply_to:
+        msg['Reply-To'] = [email.headerregistry.Address(display_name=full_name(r), addr_spec=r.email) for r in reply_to]
+
     if dest.is_teacher and target:
         target_uid = target.uid
         title = f"{topic.title} / {full_name(target)}"