provide a default JS handler for confirm-delete in crud-list

- vanilla JS since jQuery hasn't normally loaded at that point
- in an overridable block

fixes #117

keg_auth/templates/keg_auth/crud-list.html

@@ -26,4 +26,18 @@ <h1>{{ page_heading }}</h1>
     </div>
     {{ grid.html() | safe}}
 </section>
+
+{% block confirm_delete_script %}
+<script type="text/javascript">
+    var els = document.getElementsByClassName('confirm-delete');
+
+    for (var idx = 0; idx < els.length; idx++) {
+        els[idx].addEventListener('click', function (event) {
+            if (!confirm('Are you sure you want to delete this item?')) {
+                event.preventDefault();
+            }
+        });
+    }
+</script>
+{% endblock %}
 {% endblock %}