Validation of the Proposition class
Validation of the Comment class
Bug correction: nothing to validate with the slug
Validation of the Thematique class and modification of the README
Validation of the User class
Proposition class: validation of the unicity of the id and the slug
Thematique class: validation of the unicity of the id and the slug
User class: validation of the unicity of the id
Comment class: validation of the unicity of the id