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/data-retention/edit.html

@@ -1,7 +1,7 @@
 {% extends "withnav_template.html" %}
 {% from "components/textbox.html" import textbox %}
 {% from "components/page-footer.html" import page_footer %}
-
+{% from "components/form.html" import form_wrapper %}
 {% from "components/radios.html" import radios %}
 
 {% block service_page_title %}
@@ -12,7 +12,7 @@
 
 
   <h1 class="heading-large">Set data retention</h1>
-    <form method="post">
+    {% call form_wrapper() %}
       {{ notification_type | capitalize}}
       {{  textbox(form.days_of_retention) }}
       {{ page_footer(
@@ -20,6 +20,6 @@
       back_link=url_for('.edit_data_retention', service_id=current_service.id, data_retention_id=data_retention_id),
       back_link_text='Back'
     ) }}
-    </form>
+    {% endcall %}
 
-{% endblock %}
+{% endblock %}