Log when entities.csv is downloaded

We log a `form.submission.export` event in the audit log when a submissions.csv or submissions.csv.zip is downloaded. We're thinking that we should do similar when an entities.csv is downloaded. We would log that event only from the …/entities.csv endpoint, not the form attachment endpoint requested by OpenRosa clients. Otherwise, an audit log event would be logged every time an OpenRosa client downloads entities.

Once we log the new event, we should display it in Frontend.