Cancel and refund an order with payment provider

[jonathanreveille]

Purpose

This PR will solve this issue

We want to able to cancel and then refund an order if all conditions are met. The refund state can only be accessible if the order is canceled, not free and the student has paid at least 1 installment in the payment schedule.

Proposal

• [x] Add new state 'refund' in the order flows.
• [x] Add endpoint refund for payment backends.
• [x] Add admin API endpoint to cancel and refund an order.
• [x] Fix debug views for payment and mail reminders for paid installments