[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.

PHPBB3-13050
Merge pull request #2931 from rxu/ticket/13050

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

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