WIP ✨ (backend) prepare batch of orders

[jonathanreveille]

Purpose

We want companies to be able to order in batch seats for a session.

The company will first need to sign a convention (a contract), and only when this one is signed, we can
allow to generate orders that are "ready-to-use".

Once the orders are generated, companies will receive their discount coupons of (100%) and its their responsibility to hand them out to each of their employees that figure on the convention. The company will pay after signing the convention directly to the organization.

Notes:

• all the "students" of that batch order will be affected to the same organization.
• the company will be able to pay directly to the organization.
• the company will need to list of his students he wishes to subscribe to that session.
• the signature of the convention (contract) will trigger the whole process of preparing the orders
• the student will be given a coupon of 100% to use when he purchases the offer, allowing him not to pay, nor signing a contract because it has been handled beforehand by the company and the organization.

Proposal

• prepare batch order model
• update signature backends for submitting convention
• add method to generate orders once convention is signed
• add method to generate vouchers once orders are generated
• refactor generate_document_context for contract