Added reply to subscribed thread alert.

From here: http://community.mybb.com/thread-127444-post-1126413.html#pid1126413

Users who are subscribed to a thread will receive a notification when a new reply is made. This excludes the person who made the post. A DB query is made to get the UIDs of all the subscribed users - there is no way to do this type of alert without it. However, other alerts also require queries and I think this alert is important enough to warrant an extra query.