Added Forum subscriptions to automatically subscribe to new threads in a given forum

DB Migration:

DROP TABLE `subscribed_forums`;

CREATE TABLE `users_subscribed_forums` (
  `UserID` int(10) NOT NULL,
  `ForumID` int(10) NOT NULL,
  PRIMARY KEY (`UserID`,`ForumID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin