Use service setting to determine prefixing of SMSs

Rather than doing this nasty `if` statement, let the API work out what
to do. Also means that the logic is not repeated between the two apps.

app/utils.py

@@ -288,7 +288,7 @@ def get_template(
         return SMSPreviewTemplate(
             template,
             prefix=service['name'],
-            sender=sms_sender if sms_sender else (service['sms_sender'] not in {'GOVUK', None}),
+            sender=not service['prefix_sms_with_service_name'],
             show_recipient=show_recipient,
             redact_missing_personalisation=redact_missing_personalisation,
         )