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/support/index.html

@@ -1,6 +1,7 @@
 {% extends "withoutnav_template.html" %}
 {% from "components/radios.html" import radios %}
 {% from "components/page-footer.html" import page_footer %}
+{% from "components/form.html" import form_wrapper %}
 
 {% block per_page_title %}
   Support
@@ -13,10 +14,11 @@
 
     <h1 class="heading-large">Support</h1>
 
-    <form method="post" class="bottom-gutter-2">
+    {% call form_wrapper(class="bottom-gutter-2") %}
       {{ radios(form.support_type) }}
       {{ page_footer('Continue') }}
-    </form>
+    {% endcall %}
+
     <p>If something’s wrong, you can <a href="https://status.notifications.service.gov.uk/"> check the GOV.UK Notify system status page</a> to see if we’re already aware of it.</p>
 
     <h2 class="heading-medium">24-hour support</h2>