[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

phpBB/language/en/acp/board.php

@@ -99,6 +99,7 @@ $lang = array_merge($lang, array(
 	'ALLOW_TOPIC_NOTIFY'		=> 'Allow subscribing to topics',
 	'BOARD_PM'					=> 'Private messaging',
 	'BOARD_PM_EXPLAIN'			=> 'Enable private messaging for all users.',
+	'ALLOW_BOARD_NOTIFICATIONS' => 'Allow board notifications',
 ));
 
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