V10: Fix to allow dragged images in the rich text editor to be correctly uploaded (#13016)

* update string extensions IsFullPath to support more filepaths with new built-in Path.IsPathFullyQualified

* resolve TODO to switch to Path.IsPathFullyQualified supported from .NET Standard 2.1

* Use content root instead of web root for uploaded images

* Un-break a breaking change

* handle special parsing of AngularJS json response

* change htmlId selector to support html id's with numbers

* remove bad test case

* test IsFullPath without tricky UNC paths that are not useful

Co-authored-by: Bjarke Berg <mail@bergmania.dk>

src/Umbraco.Core/Extensions/StringExtensions.cs

@@ -1326,11 +1326,8 @@ public static class StringExtensions
     /// <param name="path"></param>
     /// <returns></returns>
     // From: http://stackoverflow.com/a/35046453/5018
-    public static bool IsFullPath(this string path) =>
-        string.IsNullOrWhiteSpace(path) == false
-        && path.IndexOfAny(Path.GetInvalidPathChars().ToArray()) == -1
-        && Path.IsPathRooted(path)
-        && Path.GetPathRoot(path)?.Equals(Path.DirectorySeparatorChar.ToString(), StringComparison.Ordinal) == false;
+    // Updated from .NET 2.1+: https://stackoverflow.com/a/58250915
+    public static bool IsFullPath(this string path) => Path.IsPathFullyQualified(path);
 
     // FORMAT STRINGS