implement optional channel removal hooks

add configuration variables to the default config

config.js.dist

@@ -17,6 +17,15 @@ module.exports = {
      */
     logToStdout: false,
 
+    /*  Cryptpad can be configured to remove channels some number of ms
+        after the last remaining client has disconnected.
+
+        Default behaviour is to keep channels forever.
+        If you enable channel removal, the default removal time is one minute
+    */
+    removeChannels: false,
+    channelRemovalTimeout: 60000,
+
     // You now have a choice of storage engines
 
     /* amnesiadb only exists in memory.