[ticket/11444] Moving the in-board notifications to a method class

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/PHPBB3-11444
https://tracker.phpbb.com/browse/PHPBB3-11967

PHPBB3-11444

tests/mock/notification_manager.php

@@ -32,19 +32,18 @@ class phpbb_mock_notification_manager
 		);
 	}
 
-	public function mark_notifications_read()
+	public function mark_notifications()
 	{
 	}
 
-	public function mark_notifications_read_by_parent()
+	public function mark_notifications_by_parent()
 	{
 	}
 
-	public function mark_notifications_read_by_id()
+	public function mark_notifications_by_id()
 	{
 	}
 
-
 	public function add_notifications()
 	{
 		return array();