Use a macro for form tags

This will stop us repeatedly forgetting to add `novalidate` and
`autocomplete='off'` to our forms (which is how most of them are set
up).

It uses sensible defaults, based on how we most-commonly configure
forms:
- most of our forms are `post`ed (but this can be overridden)
- `autocomplete` should only be enabled where it makes sense, otherwise
  it’s more annoying than useful (but this can be overriden)
- we should never be using HTML5 form validation because our own error
  styles and messages are better

app/templates/views/service-settings/delete.html

@@ -1,5 +1,6 @@
 {% extends "withnav_template.html" %}
 {% from "components/page-footer.html" import page_footer %}
+{% from "components/form.html" import form_wrapper %}
 
 {% block service_page_title %}
   Delete service
@@ -33,13 +34,13 @@
         </li>
       </ul>
 
-      <form method="post">
+      {% call form_wrapper() %}
         {{ page_footer(
           'Yes, delete ‘{}’'.format(current_service.name),
           destructive=True,
           back_link=url_for('.service_settings', service_id=current_service.id)
         ) }}
-      </form>
+      {% endcall %}
 
     </div>
   </div>