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/user-profile/authenticate.html

@@ -1,6 +1,7 @@
 {% extends "withoutnav_template.html" %}
 {% from "components/textbox.html" import textbox %}
 {% from "components/page-footer.html" import page_footer %}
+{% from "components/form.html" import form_wrapper %}
 
 {% block per_page_title %}
   Change your {{ thing }}
@@ -13,13 +14,13 @@
   <div class="grid-row">
     <div class="column-three-quarters">
 
-    <form method="post">
+    {% call form_wrapper(autocomplete=True) %}
       {{ textbox(form.password) }}
       {{ page_footer(
         'Confirm',
         back_link=back_link
       ) }}
-    </form>
+    {% endcall %}
     </div>
   </div>