Show all pages of a letter on the preview page

We’ve had a support ticket saying:
> Hi, where a letter goes over to two sides, is there a way in the
> 'Preview' screen (or anywhere else) that I can see page two? I can
> see page one OK, but can't work out how to see what's generated on the
> second page.

Whether you’re about to send 1000s of letters – or just want to preview
how one will look – it’s probably useful to be able to see more than
just the first page.

app/main/views/send.py

@@ -547,6 +547,7 @@ def _check_messages(service_id, template_id, upload_id, preview_row, letters_as_
         ) if not letters_as_pdf else None,
         email_reply_to=email_reply_to,
         sms_sender=sms_sender,
+        page_count=get_page_count_for_letter(db_template),
     )
     recipients = RecipientCSV(
         contents,
@@ -661,7 +662,7 @@ def check_messages_preview(service_id, template_id, upload_id, filetype, row_ind
     template = _check_messages(
         service_id, template_id, upload_id, row_index, letters_as_pdf=True
     )['template']
-    return TemplatePreview.from_utils_template(template, filetype)
+    return TemplatePreview.from_utils_template(template, filetype, page=request.args.get('page', 1))
 
 
 @main.route("/services/<service_id>/start-job/<upload_id>", methods=['POST'])