[#6249] ticket:447 Use a stable Sender: header in email notifications
[#6249] ticket:447 Fix test due to changes in notification email headers

Remove sender from send_digest and send_summary, since
they sent with 'noreply@sf.net' as from address,
which is stable already.
[#6249] ticket:453 Add test and docstring for Application.email_address
[#6249] ticket:464 Pass sender to direct sendmail call