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
Create a very basic user loader class to handle querying/storing
user data in a centralized location.
Use DIC collection service for notification types/methods.
Cleanup unused dependencies.
Fix some other issues.
PHPBB3-11103
