[setup] make niceurl testing more accurate, see #1024

ext/setup/script.js

@@ -0,0 +1,23 @@
+document.addEventListener('DOMContentLoaded', () => {
+    const checkbox = document.getElementById('nice_urls');
+    const out_span = document.getElementById('nicetest');
+    
+    if(checkbox !== null && out_span !== null) {
+        checkbox.disabled = true;
+        out_span.innerHTML = '(testing...)';
+
+        fetch(document.body.getAttribute('data-base-href') + "/nicetest").then(response => {
+            if(response.ok) {
+                checkbox.disabled = false;
+                out_span.innerHTML = '(test passed)';
+            }
+            else {
+                checkbox.disabled = true;
+                out_span.innerHTML = '(test failed)';
+            }
+        }).catch(() => {
+            checkbox.disabled = true;
+            out_span.innerHTML = '(request failed)';
+        });
+    }
+});