Log when entities.csv is downloaded

[matthew-white]

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.