Rename Edition::AuditTrail#last_audit_trail_version_event

I found this name confusing when reading the code. The method returns the
latest VersionAuditEntry where the `state` stored against the wrapped
PaperTrail Version object matches the state we're interested in.
Rename Edition::AuditTrail#audit_trail

I found it confusing that `Edition#audit_trail` returned an audit trail for
all editions belonging to the document. I think renaming the method to
`Edition#document_audit_trail` makes this more obvious.