Show recipient when about to send uploaded letter

The recipient of the letter now displays at the bottom of the page when
previewing a valid letter. The template preview `/precompiled/sanitise`
endpoint returns the address, but we format it to display on a single
line with commas between each line. We also need to convert the
recipient address to ASCII so that it can be stored as S3 metadata.

app/templates/views/uploads/preview.html

@@ -32,6 +32,10 @@
 
     {% if status == 'valid' and current_service.live %}
     <div class="js-stick-at-bottom-when-scrolling">
+      <p>
+        Recipient: {{ recipient }}
+      </p>
+
       <form method="post" enctype="multipart/form-data" action="{{url_for(
           'main.send_uploaded_letter',
           service_id=current_service.id,