In slack_blocks.go (line 392), refactor the IncidentOwnerInput function to accept a defaultValue and use it, similar to the functionality already implemented for the other single select input fields (Playbook and Severity).

Tricky to implement without having test Incident owners to use ---- see about creating these in the WebRCA UI before testing (we will need these to see the SelectedOption–>value to assign to the text)