Don’t show postage choice for international letters

International letters don’t have a choice of postage. Under the hood
they are either `europe` or `rest-of-world`.

So, for letters that we detect are international, this commit:
- removes the radios buttons that give users the choice of postage
- passes through either `europe` or `rest-of-world` to the API,
  depending on what address we find in the letter

This will cause the API to 500 until it can accept `europe` or
`rest-of-world` as postage types, but this is probably OK because it’s
only our services that have international letters switched on at the
moment.

app/templates/views/uploads/preview.html

@@ -52,12 +52,19 @@
       </p>
 
       {% if current_service.live %}
+        {% if international %}
+          <p class="govuk-body">
+            Postage: international
+          </p>
+        {% endif %}
         <form method="post" enctype="multipart/form-data" action="{{url_for(
             'main.send_uploaded_letter',
             service_id=current_service.id,
             file_id=file_id,
           )}}" class='page-footer'>
-            {{ radios(form.postage, hide_legend=true, inline=True) }}
+            {% if form.show_postage %}
+              {{ radios(form.postage, hide_legend=true, inline=True) }}
+            {% endif %}
             {{ page_footer("Send 1 letter") }}
         </form>
       {% endif %}