Add updated_at attribute to change event

[TristanCacqueray]

The API is doing special case when it queries change event where an event created_at is equivalent to a change updated_at. If the event had a updated_at attribute (e.g. a copy of the created_at), that would simplify the api code quite a bit.

• [ ] Add a curator function to set missing attribute on existing event
• [ ] Simplify the api

Note that this does not change the user interface.