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/new-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
@@ -17,10 +18,10 @@
       <p>
         You can now create a new password for your account.
       </p>
-      <form method="post" autocomplete="off">
+      {% call form_wrapper() %}
         {{ textbox(form.new_password, hint="At least 8 characters") }}
         {{ page_footer("Continue") }}
-      </form>
+      {% endcall %}
     {% else %}
       <p>
         Message about email address does not exist. Some one needs to figure out the words here.