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/IO/IOHelperLinux.cs

@@ -9,8 +9,6 @@ public class IOHelperLinux : IOHelper
     {
     }
 
-    public override bool IsPathFullyQualified(string path) => Path.IsPathRooted(path);
-
     public override bool PathStartsWith(string path, string root, params char[] separators)
     {
         // either it is identical to root,