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/static/templates/admin/diagnostics.hbs

@@ -352,7 +352,13 @@
         supportString += "* Reverse proxy and version: \n";
         supportString += "* Other relevant information: \n";
 
-        let jsonResponse = await fetch('{{urlpath}}/admin/diagnostics/config');
+        let jsonResponse = await fetch('{{urlpath}}/admin/diagnostics/config', {
+                'headers': { 'Accept': 'application/json' }
+        });
+        if (!jsonResponse.ok) {
+                alert("Generation failed: " + jsonResponse.statusText);
+                throw new Error(jsonResponse);
+        }
         const configJson = await jsonResponse.json();
         supportString += "\n### Config (Generated via diagnostics page)\n<details><summary>Show Running Config</summary>\n"
         supportString += "\n**Environment settings which are overridden:** {{page_data.overrides}}\n"