Add ACP option to decide who can browse board "Disable board"

At the moment a disabled board can be visited by admins, moderators and global moderators.

This is controlled here: https://github.com/phpbb/phpbb/blob/a1a6ff45557cb98a7a5861d005f66c319b14d58f/phpBB/phpbb/user.php#L372

		// Is board disabled and user not an admin or moderator?

		if ($config['board_disable'] && !defined('IN_INSTALL') && !defined('IN_LOGIN') && !defined('SKIP_CHECK_DISABLED') && !$auth->acl_gets('a_', 'm_') && !$auth->acl_getf_global('m_'))

It would be better, if the Admin could decide between those 3 options:

• only admins can access/post in disabled boards
• only admins + global moderators
• only admins + all moderators