provide "tolerant" variant for experimenting with new JSON schemas

The use case I have in mind is the following:

As the maintainer of an existing repository, you already have a JSON schema for your records, and the first thing you would like to try with invenio is to simply to plug in your JSON schema and check that you can upload a record.
After all, a JSON schema already provides a (basic) form of validation.

For a reasonably complex schema, the current version of the datamodel with loaders/serializers based on `marshmallow` and `StrictKeysMixin` forces you to write a lot of python code before you are able to achieve this rather basic milestone.

What about providing something like a `"loader/serializer": ["marshmallow", "generic"]` option for the cookiecutter, where the "generic" option would simply take in and spit out whatever the JSON schema specifies?
"generic" could also be called something like "None" or "testing" to make it clear that this is not meant for production.

Mentioning @zzacharo and @lnielsen since we were discussing this.