♻️(backend) refactor terms and conditions for contract

[jonathanreveille]

Purpose

From now on, the terms and conditions (CGV in french) must be specific for every organization. This means we can't have a global one for the whole platform. The terms and conditions will be injected into the contract's context, so the Order model does need to track down if the user has accepted those, since he will accept them while signing the contract.

Proposal

• [x] Remove field has_consent_to_terms for Order model (remove from model, remove admin django, remove from serializer)
• [x] Update the preparation of contract definition context for the section terms_and_conditions and the template of contract definition for the section terms_and_conditions

[lunika]

You have 3 items in your proposal but only one commit. This is not logical. You should split in 3 different commits.

[jbpenrath]

⚠️ You must merge this PR into https://github.com/openfun/joanie/tree/dev branch not https://github.com/openfun/joanie/tree/main !