Previously, this would include the entire User record in the update
hash, which was rendered in the response using `to_json`, erroneously
exposing every attribute of that record, including their (now removed)
private token.
Now we only include the user ID, and perform the lookup on-demand.
- Use issue/merge_request IID instead of ID
- Duplicate the original `TimeTrackingEndpoints` concern (+ specs) for V3, since
this is a breaking change.