Build endpoint CreateModel

We need the CreateModel now to be able to ingest automatically models from CI.

This endpoint supports the following fields:

• name (required)
• description (optional)

An admin membership is automatically created for the user that creates the model. Anyone logged in can create a model.

Make sure that if the model already exists with that name:

• if the user has access to it, it returns a 400 but with the model ID in the payload
• if the user has no access, it returns a 403 without the ID