Registries as data in Studio

In the context of the shorten Nuxeo platform & WebUI release cycle , we need to be able to add new target platforms and WebUI registries without having to deploy a new NOS version.

Solution is to have platform list, registries and WebUI registries as data in the application instead of having them only in xml contributions and json files.

Technical decisions are to be written in https://github.com/nuxeo/nos/pull/286