Add support for 403 subrequests to user.login route
>>> [!note] Migrated issue
<!-- Drupal.org comment -->
<!-- Migrated from issue #3508338. -->
Reported by: [mglaman](https://www.drupal.org/user/2416470)
Related to !52
>>>
<h3 id="summary-problem-motivation">Problem/Motivation</h3>
<p>We have /user/login set as our 403 path. The form looks a bit broken because the frontend theme is used and not the admin theme.</p>
<p>The route name check is broken since the route name matches the main request, not the subrequest. Not sure if this is possible but raising :)</p>
<h4 id="summary-steps-reproduce">Steps to reproduce</h4>
<p>system.site with</p>
<pre>page:<br> 403: /user/login</pre><p>go to a 403 page</p>
<h3 id="summary-proposed-resolution">Proposed resolution</h3>
<h3 id="summary-remaining-tasks">Remaining tasks</h3>
<h3 id="summary-ui-changes">User interface changes</h3>
<h3 id="summary-api-changes">API changes</h3>
<h3 id="summary-data-model-changes">Data model changes</h3>
> Related issue: [Issue #3545841](https://www.drupal.org/node/3545841)
issue