👔(backend) add `Organization` model

Purpose

We introduce the Organization model has a "hat" for all users and team.

Each User must have a "default" organization. Each Team must have an organization.

When a User creates a new Team, the team is linked to their default Organization.

For now the Organization should not be visible to end users this is a purely technical aspect as it.

The models are also adding a permission to allow User to edit an Organization, but for now there are no endpoints for that.

Add the Organization model, currently only managed by the backend.

[x] add Organization model
[x] add FK from User to Organization
[x] add FK from Team to Organization
[x] add OrganizationAccess : not used now
[x] add "siret" scope (siren is almost never present in IdP)
[x] Update authentication backend to set organization on user creation
[x] Update Team creation endpoint to set user organization on the Team
[x] before merge ask the ProConnect team to allow "siret" scope for "La Régie" SP
[x] Add siret to OIDC_RP_SCOPES environment variable in deployment config
[x] Add ORGANIZATION_REGISTRATION_ID_VALIDATORS with value '[{"NAME": "django.core.validators.RegexValidator", "OPTIONS": {"regex": "[a-z][0-9]{14}"}}]' in deployment config

Next steps:

Add an Organization to each User and Team on all environments to mark Organization as mandatory in database.
Add scope to Organization to list the Service Provider list allowed for a User in an Organization.
Add endpoints + frontend to manage Organization's scopes