#290485: Port og_notifications + integration (see #276941) to D6 + a few other...

#290485: Port og_notifications + integration (see #276941) to D6 + a few other incidental (minor) fixes to get notifications working. These include a couple of $form_values / $form_state variable changes missed during the og port; message_ids not being updated in _og_mail_text etc.

_og_mail_text also now has $language and $variables swapped to retain existing calls.