Improve handling of logout during asynchronous tasks (Z#23238978) (#6341)

* Handle logout during async task requests properly

* Handle fragments safely on webcheckin logout

* Remove unnecessary comment

* Revert hash change

src/pretix/control/templates/pretixcontrol/auth/login.html

@@ -56,5 +56,4 @@
     </form>
     <script type="text/plain" id="good_origin">{{ good_origin }}</script>
     <script type="text/plain" id="bad_origin_report_url">{{ bad_origin_report_url }}</script>
-    <!-- pretix-login-marker -->{# marker required for ajax calls to detect that user session is over #}
 {% endblock %}