Convert radios on SMS prefix page

Includes changing form.enabled to use
OnOffField, for consistency with other on/off
fields.

OnOffField's data is a boolean, not a string, so
some of the logic using it needed to be changed.

app/templates/views/service-settings/sms-prefix.html

@@ -1,5 +1,4 @@
 {% extends "withnav_template.html" %}
-{% from "components/radios.html" import radios %}
 {% from "components/page-header.html" import page_header %}
 {% from "components/page-footer.html" import page_footer %}
 {% from "components/form.html" import form_wrapper %}
@@ -16,7 +15,7 @@
   ) }}
 
   {% call form_wrapper() %}
-    {{ radios(form.enabled) }}
+    {{ form.enabled }}
     {{ page_footer('Save') }}
   {% endcall %}