disable update creation while creating a new incident

[snkaupe]

Until now, it was possible to create incident updates when creating a new incident. This leads to requests running into errors because the updates are sent to the API server before the new incident. Due to this, there is not yet an incident ID to assign these updates to and the API server responds with an error.

A proper fix would be to rewrite the code that handles the update and incident creation, but this is better done once the overall management page has been cleaned up. Therefore, for now, we just disable the fields needed to create updates when creating a new incident.