I43: Implement comments
CREATE TABLE `comments` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `user_id` char(32) DEFAULT NULL,
  `post_id` int(11) NOT NULL,
  `text` text,
  `parent_id` int(10) DEFAULT NULL,
  `created` int(10) NOT NULL,
  `edited` int(10) DEFAULT NULL,
  `deleted` int(10) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `post_id` (`post_id`,`user_id`),
  KEY `id` (`id`,`post_id`)
)