Merge pull request #5864 from Garpo001/patch-1

Serialization error is thrown when creating layout or page in the dashboard when using Layouts in a Web Farm.

src/Orchard.Web/Modules/Orchard.Layouts/Models/ElementSessionState.cs

@@ -1,4 +1,7 @@
+using System;
+
 namespace Orchard.Layouts.Models {
+        [Serializable]
         public class ElementSessionState {
         public string TypeName { get; set; }
         public string ElementData { get; set; }