Fixes back office check, adds more tests, changes area to be a constant

src/Umbraco.Web.BackOffice/Controllers/UpdateCheckController.cs

@@ -15,7 +15,7 @@ using Umbraco.Web.Security;
 
 namespace Umbraco.Web.BackOffice.Controllers
 {
-    [PluginController("UmbracoApi")]
+    [PluginController(Constants.Web.Mvc.BackOfficeApiArea)]
     public class UpdateCheckController : UmbracoAuthorizedJsonController
     {
         private readonly IUpgradeService _upgradeService;