Fix media paths with UNC format

[bergmania]

Summary

This ensures we are not calling hostingEnvironment.MapPathWebRoot when the path is already rooted.

How to test

• Set Umbraco:CMS:Global:UmbracoMediaPhysicalRootPath to an UNC path (e.g. \\ServerName\SharedFolderName\Media)
• Boot umbraco
• Upload media to media section
• Verity the media is updated to the specified path.

[p-m-j]

tested

• Windows with UNC path to samba share e.g. "\\nas\umbmedia" ✔️
• Windows with releative path e.g. "umbmedia" - ends up in wwwroot/umbmedia ✔️
• Linux absolute path e.g. "/home/pmj/umbmedia" ✔️
• Linux relative path e.g. "umbmedia" - ends up in wwwroot/umbmedia ✔️

[nikolajlauridsen]

Cherry picked for 9.3.1 💪

[ronaldbarendse]

Note that using Path.IsPathRooted() doesn't ensure an absolute path: it returns true for C:Media, but that is still relative to the current working directory (I guess that's the content root in an ASP.NET Core application). See: https://docs.microsoft.com/en-us/dotnet/api/system.io.path.ispathfullyqualified?view=net-5.0#remarks.

The same (wrong) assumption is made in the PhysicalFileSystem:

Umbraco-CMS/src/Umbraco.Core/IO/PhysicalFileSystem.cs

Lines 45 to 54 in 8eb3ef6

	// rootPath should be... rooted, as in, it's a root path!
	if (Path.IsPathRooted(rootPath) == false)
	{
	// but the test suite App.config cannot really "root" anything so we have to do it here
	var localRoot = hostingEnvironment.MapPathContentRoot("~");
	rootPath = Path.Combine(localRoot, rootPath);
	}
	// clean up root path
	rootPath = Path.GetFullPath(rootPath);

Using Path.IsPathFullyQualified() seems more appropriate.

[p-m-j]

It certainly seems that IsPathFullyQualified aligns better with the original intent in PhysicalFileSystem according to the comments, but I don't think it matters too much.

var localRoot = @"C:\Users\Paul Johnson\Dev\umbraco\v9\src\Umbraco.Web.UI";
var physicalMediaPath = "C:media";
var combined = Path.Combine(localRoot, physicalMediaPath);
Path.GetFullPath(combined) == Path.GetFullPath(physicalMediaPath); // true