Create On Call List

A new list. The time zone falls back to the organization's.

Without on_call_now_user_id, the first member's turn is the one starting at first_handoff_local, a clock time in the list's time zone with no offset.

One list as it stands at a moment.

backup is the next member after whoever's regular turn it is, skipping whoever is on call. swaps are the ones running or still to come. warnings holds fewer_than_two when the list has no backup. rule_keys are the rules whose newest or active version names the list.