Don't force payment for backoffice pending contribution

When creating a backoffice contribution, Payment Method is currently a required field, and Payment Received datetime defaults to now.

1. Payment Method should only be required when contribution status is Completed, Partially paid, or pending refund, and not otherwise.

2. When Contribution Status is changed to Pending or Failed, Payment Received should be set to null on screen.

3. If Payment Received is null, and Contribution Status is changed to a value other than Pending or Failed, Payment Received should be defaulted to now.