Add an activity_timestamp column in participants table

docs/schema.mysql

@@ -24,6 +24,7 @@ CREATE TABLE `participants` (
   `participant` varchar(60) NOT NULL,
   `peer_id` varchar(40) DEFAULT NULL,
   `role` varchar(20) DEFAULT 'participant',
+  `activity_timestamp` int(20) DEFAULT NULL,
   PRIMARY KEY (`id`,`participant`)
 ) DEFAULT CHARSET=utf8;
 DROP TABLE IF EXISTS `notifications`;

docs/upgrade.mysql

@@ -22,3 +22,4 @@ CREATE TABLE IF NOT EXISTS `invitations` (
   PRIMARY KEY (`token`)
 ) DEFAULT CHARSET=utf8;
 ALTER TABLE `rooms` ADD COLUMN `etherpad_group` varchar(40) DEFAULT NULL AFTER `owner_password`;
+ALTER TABLE `participants` ADD COLUMN `activity_timestamp` int(20) DEFAULT NULL AFTER `role`;