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/invite-user.html

@@ -2,6 +2,7 @@
 {% from "components/checkbox.html" import checkbox %}
 {% 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 %}
   Invite a team member
@@ -14,7 +15,7 @@
   </h1>
 
   <div class="grid-row">
-    <form method="post" class="column-three-quarters" novalidate>
+    {% call form_wrapper() %}
 
       {{ textbox(form.email_address, width='1-1', safe_error_message=True) }}
 
@@ -22,6 +23,6 @@
 
       {{ page_footer('Send invitation email') }}
 
-    </form>
+    {% endcall %}
   </div>
 {% endblock %}