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/send-test.html

@@ -2,6 +2,7 @@
 {% from "components/page-footer.html" import page_footer %}
 {% from "components/message-count-label.html" import recipient_count_label %}
 {% from "components/textbox.html" import textbox %}
+{% from "components/form.html" import form_wrapper %}
 
 {% block service_page_title %}
   {{ page_title }}
@@ -13,7 +14,7 @@
     {{ page_title }}
   </h1>
 
-  <form method="post" class="js-stick-at-top-when-scrolling" data-module="autofocus" autocomplete="off" novalidate>
+  {% call form_wrapper(class="js-stick-at-top-when-scrolling", module="autofocus") %}
     <div class="grid-row">
       <div class="column-two-thirds {% if form.placeholder_value.label.text == 'phone number' %}extra-tracking{% endif %}">
         {{ textbox(
@@ -36,7 +37,7 @@
       </p>
     {% endif %}
     {{ page_footer('Continue', back_link=back_link) }}
-  </form>
+  {% endcall %}
 
   {{ template|string }}