Send Change History to Content API

With the change notes, we were only persisting the latest change note. This commit inserts a hash of the timestamp and change note into a change history Array and sends this to the content API.

It also deals with minor update checkbox and major update change note persistence between versions.
Or even

```
document.change_history << latest_change_history unless document.minor_update
```

to avoid changing state "magically". Though I'd be fine with either approach.