refactored IUpgradeable to use the migration attribute approach

Oqtane.Server/Infrastructure/Interfaces/SiteUpgradeAttribute.cs

@@ -0,0 +1,27 @@
+using System;
+
+namespace Oqtane.Infrastructure
+{
+    [AttributeUsage(AttributeTargets.Class)]
+    public class SiteUpgradeAttribute : Attribute
+    {
+        private string aliasname;
+        private string version;
+
+        public SiteUpgradeAttribute(string AliasName, string Version)
+        {
+            aliasname = AliasName;
+            version = Version;
+        }
+
+        public virtual string AliasName
+        {
+            get { return aliasname; }
+        }
+
+        public virtual string Version
+        {
+            get { return version; }
+        }
+    }
+}