Added document max length limit, enforceMaxLength on change and show modal when reach the limit.

lib/ot/editor-socketio-server.js

@@ -95,6 +95,7 @@ EditorSocketIOServer.prototype.onOperation = function (socket, revision, operati
     try {
         var clientId = socket.id;
         var wrappedPrime = this.receiveOperation(revision, wrapped);
+        if(!wrappedPrime) return;
         //console.log("new operation: " + JSON.stringify(wrapped));
         this.getClient(clientId).selection = wrappedPrime.meta;
         revision = this.operations.length;