Changes to Basic Auth to support external logins (#12434)

* Fixed issues with basic auth middleware to support Umbraco Cloud usecase

* Fix redirects to return url, now allows website urls

* Strip potential domain part of returnPath

* Fix default value in appsettings schema

* Reintroduce check of basic auth enabled.

* Fix wrong negation introduced in #12349

* Fixed issues with redirects

* Also check external login cookie, while authenticating backoffice

src/Umbraco.Web.BackOffice/Controllers/BackOfficeController.cs

@@ -322,7 +322,7 @@ namespace Umbraco.Cms.Web.BackOffice.Controllers
         [AllowAnonymous]
         public ActionResult ExternalLogin(string provider, string? redirectUrl = null)
         {
-            if (redirectUrl == null)
+            if (redirectUrl == null || Uri.TryCreate(redirectUrl, UriKind.Absolute, out _))
             {
                 redirectUrl = Url.Action(nameof(Default), this.GetControllerName());
             }