Show recipient on notification page

Since we removed the notifications table from this page we need a
different way of showing who the message was sent to.

Our `Template` classes already have a way of doing this, and we have a
flag to switch this on (which is what this PR changes).

app/main/views/notifications.py

@@ -58,6 +58,7 @@ def view_notification(service_id, notification_id):
                 version=notification['template_version'],
                 filetype='png',
             ),
+            show_recipient=True,
         ),
         updates_url=url_for(
             ".view_notification_updates",