Actually use jinja2 formatting for if

templates/admin.html

@@ -36,7 +36,7 @@
                                 <input type="submit" name="expire" value="Expire">
                             {% else %}
                                 <input type="submit" name="delete"
-                                           value="Delete" {% disabled="true" if registration_code.usages > 0 %}>
+                                       value="Delete" {% if registration_code.usages > 0 %} disabled="true" {% endif %}>
                             {% endif %}
                         </form>
                     </td>