:sparkles: (backend) admin api endpoint for batch order

[jonathanreveille]

Purpose

We need actions for the batch order through the admin API endpoints. We will add the send invitation signature link, validation of a successful payment, the generation of orders and the action to send an email with the voucher codes to the owner of the batch order.

Actually, the admin api batch order viewset allow to create a batch order, either with a specific organizaition, or we get the organization with the least orders. With these new endpoints, admin user will be able to : create invitation link to sign contract, validate payment if it's done outside the payment backend, generate orders, send voucher codes by email.

Proposal

• [x] add admin api endpoint to validate payment
• [x] add admin api endpoint to generate orders
• [x] add admin api endpoint to send voucher codes
• [x] add admin api endpoint to send invitation link signature