timeline: Own edits don't show up in `latest_edit_json`

[VAWVAW]

Steps to reproduce:

1. find an unedited message in the timeline
2. edit the event using Timeline::edit()
3. look at the EventTimelineItem::latest_edit_json() for the replacement event

Expected output

The new content is shown.

Current output

The field is None.

Note

This can also be observed in ElementX.