b2f16d6ab72213cd28f203af5e0b34ad52d7220b

Added: opening_id changes the offer's opening, or removes it when sent as null if your organization doesn't require one. The change creates a new offer version when your organization versions offers on opening changes.
Fixed: A starts_on change that creates a new offer version keeps the offer's opening_id instead of clearing it.
Fixed: A request where starts_on and another version-triggering change, such as a custom field, would each create a new offer version now creates one version instead of two.

Added: opening_id sets the opening the offer is for. It must be an open opening on the application's job that no other unresolved offer holds, and it's required when your organization requires an opening on offers.
Fixed: Organizations that require an opening on offers can create offers. The request no longer fails with a 422 asking for the Opening custom field, which callers couldn't set.