Show text message sender in send one-off flow

If you’ve chosen a text message sender then it’s good to see
confirmation of your choice.

This replicates what we do when you choose an email reply-to address.

app/utils.py

@@ -272,7 +272,7 @@ def get_template(
     page_count=1,
     redact_missing_personalisation=False,
     email_reply_to=None,
-    sms_sender=None
+    sms_sender=None,
 ):
     if 'email' == template['template_type']:
         return EmailPreviewTemplate(
@@ -290,6 +290,7 @@ def get_template(
             prefix=service['name'],
             show_prefix=service['prefix_sms'],
             sender=sms_sender,
+            show_sender=bool(sms_sender),
             show_recipient=show_recipient,
             redact_missing_personalisation=redact_missing_personalisation,
         )