Provide Bulk Issue Update metadata in webhook events

Currently webhooks are fired for bulk updates, perhaps that is ok, perhaps not?

If the events do arrive, remote webhook clients have no way to know the status of the 'bulk update' and the selection (or not) of the 'dont send email' checkbox.

The in-VM IssueEvent does have this property available, so at the least, it should be present in the webhook data, or the event should be filtered at source?