Switch to calling Payment.create api when processing a refund from AdditionalPayment...

Switch to calling Payment.create api when processing a refund from AdditionalPayment form

This is in conjunction with a series of changes consolidating & adding testing for creating payments with the goal
being that the logic is all in the Payment.create api/BAO & none of the logic remains in the CRM_Contribute_Form_AdditionalPayment form

This change completes switching the refund action over. There is logic in there to update the Participant status to
Registered when a refund is processed - seemingly regardless of what it's original status is or the appropriate status
afterwards.

I tested through the UI and if you create a participant record & then change the line items, thus reducing the cost
and changing the contribution status to 'Pending Refund' the participant status remains 'Completed' so I think
this was not working / not required already & I simply dropped it