V13: The login screen does not display external login errors

[iOvergaard]

Description

This fixes a regression compared from V12 to V13 where the new login screen UI did not show external login errors that happened on the server side. The is two-fold: BackOfficeController needed to have its built-in error messages migrated from V12, and the UmbracoLogin/Index.cshtml needed to hook on to those errors and forward them to the login screen UI.

Fixes #15622

How to test

Set a cookie containing external login errors somewhere, e.g. in the BackOfficeController:

var someerrormessage = @"{""errors"":[""Some error message""]}";
_httpContextAccessor.Response.Cookies.Append(ViewDataExtensions.TokenExternalSignInError, Convert.ToBase64String(Encoding.UTF8.GetBytes(someerrormessage)), new CookieOptions() { HttpOnly = true, Secure = true });

Verify that the error message shows up on the login screen below the welcome message.

[Migaroez]

Looks good and tests good!

[nul800sebastiaan]

@iOvergaard This broke the user panel in the backoffice - I can't see from a quick glance exactly what is wrong but this is how it looks in the RC now:

Error:

angular.js:15697 Error: [$injector:unpr] http://errors.angularjs.org/1.8.3/$injector/unpr?p0=externalLoginInfoProvider%20%3C-%20externalLoginInfo%20%3C-%20externalLoginInfoService
    at angular.js:99:1
    at angular.js:4991:19
    at Object.d [as get] (angular.js:5151:32)
    at angular.js:4996:28
    at d (angular.js:5151:32)
    at e (angular.js:5176:58)
    at Object.invoke (angular.js:5200:18)
    at Object.$get (angular.js:5043:20)
    at Object.invoke (angular.js:5208:16)
    at angular.js:4997:20 '<div ng-if="model.view &amp;&amp; !model.animating &amp;&amp; !model.$parentScope" ng-include="model.view" class="ng-scope">'