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/forgot-password.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 %}
 Create a new password
@@ -14,10 +15,10 @@ Create a new password
 
     <p>We’ll send you an email to create a new password.</p>
 
-    <form autocomplete="off" method="post" novalidate>
+    {% call form_wrapper() %}
       {{ textbox(form.email_address, safe_error_message=True) }}
       {{ page_footer("Send email") }}
-    </form>
+    {% endcall %}
 
   </div>
 </div>