[RFC] Deprecate ReST API

[jordisala1991]

Subject

I am targeting this branch, because this is BC.

Changelog

### Deprecated
- Deprecated ReST API with FOSRest, Nelmio Api Docs and JMS Serializer.

[jordisala1991]

With this deprecation there are many things that are solved.

1. We stop offering an API that becomes outdated very quickly since each version of Nelmio changes completely the annotations
2. We can remove a lot of dependencies: fosRest, nelmio, JMS serializer. In fact, on master with this PR we would become only dependants on symfony, our own bundles, gauffrete (only php dependency) and imagine (only php dependency) for the require part.
3. We don't have to worry about Datagrid-bundle anymore because on 4.0 it won't be needed.

I think we should take this PR as the way to go for the rest of bundles and apply the same. wdyt @sonata-project/contributors ?

[phansys]

IMO, the title and the changelog must also reflect that we are talking specifically about the ReST API.

[VincentLanglet]

Does someone in the @sonata-project/contributors is using the Rest Api ?

[jordisala1991]

3 approvals + @phansys did an approval before. Lets do this.