Add the download activity link for notifications.

This works locally for a long running request and a large number of messages. However I suspect that nginx may be timing out the request. I'd like to try this on staging.

app/templates/views/notifications.html

@@ -47,6 +47,11 @@
       <input type="hidden" name="csrf_token" value="{{ csrf_token() }}"/>
     </form>
   {% endif %}
+  <p class="bottom-gutter">
+     <a href="{{ download_link }}" download="download" class="heading-small">Download this report</a>
+     &emsp;
+     Data available for 7 days
+  </p>
   {{ ajax_block(
     partials,
     url_for('.get_notifications_as_json', service_id=current_service.id, message_type=message_type, status=status, page=page),