redirect to admin login page when forward fails

currently, if the admin guard fails the user will get a 404 page.
and when the session times out after 20 minutes post methods will
give the reason "undefined" as a response while generating the support
string will fail without any user feedback.

this commit changes the error handling on admin pages

* by removing the reliance on Rockets forwarding and making the login
  page an explicit route that can be redirected to from all admin pages

* by removing the obsolete and mostly unused Referer struct we can
  redirect the user back to the requested admin page directley

* by providing an error message for json requests the
  `get_diagnostics_config` and all post methods can return a more
  comprehensible message and the user can be alerted

* the `admin_url()` function can be simplified because rfc2616 has been
  obsoleted by rfc7231 in 2014 (and also by the recently released
  rfc9110) which allows relative urls in the Location header.

  c.f. https://www.rfc-editor.org/rfc/rfc7231#section-7.1.2 and
  https://www.rfc-editor.org/rfc/rfc9110#section-10.2.2

src/main.rs

@@ -435,6 +435,7 @@ async fn launch_rocket(pool: db::DbPool, extra_debug: bool) -> Result<(), Error>
         .mount([basepath, "/notifications"].concat(), api::notifications_routes())
         .register([basepath, "/"].concat(), api::web_catchers())
         .register([basepath, "/api"].concat(), api::core_catchers())
+        .register([basepath, "/admin"].concat(), api::admin_catchers())
         .manage(pool)
         .manage(api::start_notification_server())
         .attach(util::AppHeaders())