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-user-permissions.html

@@ -1,6 +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 %}
 
 {% block service_page_title %}
   {{ user.name or user.email_localpart }}
@@ -17,7 +18,7 @@
   </p>
 
   <div class="grid-row">
-    <form method="post" class="column-three-quarters">
+    {% call form_wrapper(class="column-three-quarters") %}
 
       {% include 'views/manage-users/permissions.html' %}
 
@@ -27,6 +28,6 @@
           delete_link_text='Remove user from service'
         ) }}
 
-    </form>
+    {% endcall %}
   </div>
 {% endblock %}