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/main/views/service_settings.py

@@ -630,15 +630,13 @@ def service_set_inbound_number(service_id):
 @user_has_permissions('manage_service')
 def service_set_sms_prefix(service_id):
 
-    form = SMSPrefixForm(enabled=(
-        'on' if current_service.prefix_sms else 'off'
-    ))
+    form = SMSPrefixForm(enabled=current_service.prefix_sms)
 
     form.enabled.label.text = 'Start all text messages with ‘{}:’'.format(current_service.name)
 
     if form.validate_on_submit():
         current_service.update(
-            prefix_sms=(form.enabled.data == 'on')
+            prefix_sms=form.enabled.data
         )
         return redirect(url_for('.service_settings', service_id=service_id))