auth: Render config error page on configuration error.

We previously used to to redirect to config error page with
a different URL. This commit renders config error in the same
URL where configuration error is encountered. This way when
conifguration error is fixed the user can refresh to continue
normally or go back to login page from the link provided to
choose any other backend auth.

Also moved those URLs to dev_urls.py so that they can be easily
accessed to work on styling etc.

In tests, removed some of the asserts checking status code to be 200
as the function `assert_in_success_response` does that check.

zproject/dev_urls.py

@@ -9,7 +9,7 @@ from django.urls import path
 from django.views.generic import TemplateView
 from django.views.static import serve
 
-from zerver.views.auth import login_page
+from zerver.views.auth import config_error, login_page
 from zerver.views.development.email_log import clear_emails, email_page, generate_all_emails
 from zerver.views.development.integrations import (
     check_send_webhook_fixture_message,
@@ -72,6 +72,11 @@ urls = [
          send_all_webhook_fixture_messages),
     path('devtools/integrations/<integration_name>/fixtures',
          get_fixtures),
+
+    path('config-error/<error_category_name>', config_error,
+         name='config_error'),
+    path('config-error/remoteuser/<error_category_name>',
+         config_error),
 ]
 
 # Serve static assets via the Django server