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/TreeControllerBase.cs

@@ -173,7 +173,7 @@ namespace Umbraco.Web.Trees
         /// <returns></returns>
         protected virtual TreeNode CreateRootNode(FormDataCollection queryStrings)
         {
-            var rootNodeAsString = Constants.System.Root.ToString(CultureInfo.InvariantCulture);
+            var rootNodeAsString = Constants.System.RootString;
             var currApp = queryStrings.GetValue<string>(TreeQueryStringParameters.Application);
 
             var node = new TreeNode(