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-prefix.html

@@ -1,6 +1,7 @@
 {% extends "withnav_template.html" %}
 {% from "components/radios.html" import radios %}
 {% from "components/page-footer.html" import page_footer %}
+{% from "components/form.html" import form_wrapper %}
 
 {% block service_page_title %}
   Text messages start with service name
@@ -9,13 +10,13 @@
 {% block maincolumn_content %}
 
   <h1 class="heading-large">Text messages start with service name</h1>
-  <form method="post">
+  {% call form_wrapper() %}
     {{ radios(form.enabled) }}
     {{ page_footer(
       button_text="Save",
       back_link=url_for('.service_settings', service_id=current_service.id),
       back_link_text='Back to settings'
     ) }}
-  </form>
+  {% endcall %}
 
 {% endblock %}