Rewrite mail notifications to use the current DokuWiki mail API

This updates the discussion plugin to use the Mailer class that was
introduced in DokuWiki version 2012-01-25. This also uses the newer
(since DokuWiki version 2013-05-10) subscription API to get the
subscribed users if available.
Avoid an undefined variable
Fix mail template replacements
Fix the unsubscribe link in notifications for registered users
New version 2013-10-29 released

Changes:
  * Use the current mailer API of DokuWiki, this fixes problems with
    PHP installations with disabled /e modifier
  * Fix the unsubscribe link in notification mails for logged in users
  * Re-implemented the enable/disable handling, comments are disabled
    automatically now when the discussion syntax is removed and
    automatic discussions are disabled.
  * Translation updates