Swapped all System.Root string conversions

to use the `System.RootString` constant.

This saves on integer parsing and reduces the number of
new strings being created.

Since the `System.Root` is `-1`, it didn't need the culture invariant
conversion.

src/Umbraco.Web/Trees/ContentBlueprintTreeController.cs

@@ -44,7 +44,7 @@ namespace Umbraco.Web.Trees
             var entities = Services.EntityService.GetChildren(Constants.System.Root, UmbracoObjectTypes.DocumentBlueprint).ToArray();
 
             //check if we're rendering the root in which case we'll render the content types that have blueprints
-            if (id == Constants.System.Root.ToInvariantString())
+            if (id == Constants.System.RootString)
             {
                 //get all blueprint content types
                 var contentTypeAliases = entities.Select(x => ((ContentEntitySlim) x).ContentTypeAlias).Distinct();
@@ -91,7 +91,7 @@ namespace Umbraco.Web.Trees
         {
             var menu = new MenuItemCollection();
 
-            if (id == Constants.System.Root.ToInvariantString())
+            if (id == Constants.System.RootString)
             {
                 // root actions
                 menu.Items.Add<ActionNew>(Services.TextService, opensDialog: true);