DocumentChanged webhook _rev empty

I suspect this is down to the same root cause as CBG-522, so perhaps doing that first would resolve this.

The "_rev" property of a document_changed webhook is empty in 2.7.0. This works in 2.6.0.

Steps to reproduce

1. Start SG with a document_changed webhook set up in the config
2. Create a document
3. Update the document
4. Observe that the webhook body sent in step 3 did not contain the rev id

2.6.0:

2.7.0: