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/service-settings/sms-sender/edit.html

@@ -3,6 +3,7 @@
 {% from "components/textbox.html" import textbox %}
 {% from "components/checkbox.html" import checkbox %}
 {% from "components/page-footer.html" import page_footer %}
+{% from "components/form.html" import form_wrapper %}
 
 {% block service_page_title %}
   Edit text message sender
@@ -24,7 +25,7 @@
       Edit text message sender
     </h1>
   {% endif %}
-  <form method="post">
+  {% call form_wrapper() %}
     {% if inbound_number %}
       <p>
         <span class="bottom-gutter-1-3"> {{ sms_sender.sms_sender }} </span>
@@ -66,6 +67,6 @@
         ) }}
       {% endif %}
     {% endif %}
-  </form>
+  {% endcall %}
 
 {% endblock %}