Change the admin template logic for disabling the delete button

templates/admin.html

@@ -35,8 +35,8 @@
                                 {% if not registration_code.is_expired() %}
                                 <input type="submit" name="expire" value="Expire">
                                 {% else %}
-                                    <input type="submit" name="delete" value="Delete"
-                                           disabled="{{ True if registration_code.usages > 0 else False }}">
+                                    <input type="submit" name="delete"
+                                           value="Delete" {% disabled="true" if registration_code.usages > 0 %}>
                                 {% endif %}
                             </form>
                         </td>