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/edit-email-template.html

@@ -2,6 +2,7 @@
 {% from "components/textbox.html" import textbox %}
 {% from "components/page-footer.html" import page_footer %}
 {% from "components/radios.html" import radios %}
+{% from "components/form.html" import form_wrapper %}
 
 {% block service_page_title %}
   {{ heading_action }} email template
@@ -13,7 +14,7 @@
       {{ heading_action }} email template
     </h1>
 
-    <form method="post">
+    {% call form_wrapper() %}
       <div class="grid-row">
         <div class="column-five-sixths">
           {{ textbox(form.name, width='1-1', hint='Your recipients won’t see this', rows=10) }}
@@ -36,6 +37,6 @@
           {% endif %}
         </aside>
       </div>
-    </form>
+    {% endcall %}
 
 {% endblock %}