Update cornice and validation

We are currently using old versions of cornice and colander with custom patches (see requirements.txt)

If possible, we should not depend on 'custom' builds of third party libraries, and at least, we should update them.

Here, the PRs haven't been merged in the projects. They relate to using colander with cornice to validate the json in the request body.

Maybe a good replacement candidate could be to use cornic with marshmallow.

Start with requirements.txt to inspect the custom cornice and colander builds and the pinned versions. Review the upstream PRs mentioned in the issue, then compare updating those dependencies with replacing cornice/colander validation using marshmallow; done means request-body JSON validation works without custom third-party builds.

Written by the indexing model from the issue text.