introduced a ModelBase to move the IAuditable properties to a base class

Oqtane.Shared/Models/ModelBase.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace Oqtane.Models
+{
+    public class ModelBase : IAuditable
+    {
+        public string CreatedBy { get; set; }
+        public DateTime CreatedOn { get; set; }
+        public string ModifiedBy { get; set; }
+        public DateTime ModifiedOn { get; set; }
+    }
+}