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
Now, if there is no row for the user in the user_notifications table,
the user will receive basic notifications. If the user wishes to not
receive notifications, a row must be added with notify = 0.
For other methods besides the basic (e.g. email, jabber) a row must
still be added with notify = 1 for them to receive notifications
PHPBB3-11103
