Currently the in-board method for the notifications is hardcoded and
cannot be disabled. This method should be in his own class extending
`phpbb\notification\method\method_interface`.
It also add the possibility, for each method, to be enabled by default (ie:
no entry in the DB => notification enabled).
https://tracker.phpbb.com/browse/PHPBB3-11444https://tracker.phpbb.com/browse/PHPBB3-11967
PHPBB3-11444
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
