Do not send notification when the source document is a version

Sending notifications on version is not really relevant: it's more interesting to use a distinct event for actions related to version.

For instance, having a notification when a version is created can be a bit confusing. It's more interesting to have a notification working on documentCheckedIn, where you'll be able to have a specific message (and not the standard message about document creation)

So we'll use a veto to disable notifications on versions