Updates the owin startup classes - we now have a default one shipped as a DLL which will always execute based on the appSettings, then we can ship with 2 optional ones that people can learn and use from which just requires them to update the appSetting. Now to decide on how to ship these .cs files

src/Umbraco.Web.UI/web.Template.config

@@ -51,7 +51,7 @@
     <add key="autoFormsAuthentication" value="false" />
 
     <add key="log4net.Config" value="config\log4net.config" />
-    <add key="owin:appStartup" value="UmbracoStartup" />
+    <add key="owin:appStartup" value="DefaultUmbracoStartup" />
   </appSettings>
   <connectionStrings>
     <remove name="umbracoDbDSN" />