include theme resources on server page load, add IUpgradeable interface to provide site-based versioning support

Oqtane.Server/Infrastructure/SiteUpgrade/ExampleUpgrade.cs

@@ -0,0 +1,40 @@
+using Oqtane.Models;
+using Oqtane.Documentation;
+
+namespace Oqtane.Infrastructure
+{
+    [PrivateApi("Mark Site-Template classes as private, since it's not very useful in the public docs")]
+    public class ExampleUpgrade : IUpgradeable
+    {
+        string IUpgradeable.GetVersions(Alias alias)
+        {
+            var versions = "";
+            switch (alias.Name)
+            {
+                case "localhost:44357":
+                    // return the list of official release versions for the specific site
+                    versions = "1.0.0";
+                    break;
+            }
+            return versions;
+        }
+
+        bool IUpgradeable.Upgrade(Alias alias, string version)
+        {
+            bool success = true;
+            switch (alias.Name)
+            {
+                case "localhost:44357":
+                    switch (version)
+                    {
+                        case "1.0.0":
+                            // execute some version-specific upgrade logic for the site here such as adding pages, modules, content, etc...
+                            success = true;
+                            break;
+                    }
+                    break;
+            }
+            return success;
+        }
+    }
+}