make userlist change notifications configurable

customize.dist/application_config.js

@@ -12,6 +12,7 @@ define(function() {
      *  You can change their duration here (measured in milliseconds)
      */
     config.notificationTimeout = 5000;
+    config.disableUserlistNotifications = false;
 
     config.enablePinning = true;
 

www/common/toolbar3.js

@@ -886,6 +886,7 @@ define([
                 // type : 1 (+1 user), 0 (rename existing user), -1 (-1 user)
                 if (typeof name === "undefined") { return; }
                 name = name || Messages.anonymous;
+                if (Config.disableUserlistNotifications) { return; }
                 switch(type) {
                     case 1:
                         Cryptpad.log(Messages._getKey("notifyJoined", [name]));