[ticket/13050] Allow topic/forum subscription when email/jabber is off

Currently, there's impossible to subscribe topic/forum if emails and/or jabber
is disabled. This seems to be a 3.0 leftover since 3.1 introduces new
notification system which handles this case and offers email/jabber-free
board notifications.

<a href="https://tracker.phpbb.com/browse/PHPBB3-13050">PHPBB3-13050</a>.