[ticket/11488] Use correct base class in email notification method

In ticket/11451 this was not correctly changed to reflect the new class
phpbb_notifcation_method_messenger_base. Additionally, an undefined
variable error has been fixed in messenger_base.php (change should be
confirmed by bantu though).

PHPBB3-11488
Merge remote-tracking branch 'marc1706/ticket/11488' into develop

* marc1706/ticket/11488:
  [ticket/11488] Use correct base class in email notification method