These types are the same, except for Database.Empty

I managed to have backwards compatibility with the frontend format,
hence the frontend doesn't need any changes.

This simplifies the WithQuery structure even more

There is now only a 'datafield' field, no need for duplicated
'database'.

Related to #441

I want to understand ngrams algorithms better.

Also, add -Werror to tests so that no warnings in tests are reported anymore.

This commit adds three major tags to the swagger UI:

* Gargantext (like before): this spells out all the endpoints;
* Private operations: lists private backend ops;
* Public  operations: lists public backend ops.

More can be added, but the machinery used here should serve as a
blueprint.

OpenAPI3 will allow us to properly pass the Bearer Auth to the servant
swagger UI.

Those definitions felt very ad hoc, and were not used a lot in the
codebase, so I simply replaced them with their definition.

This reverts commit ff00345c