Fix API DoctrineORMSerializationType to be loaded only when using ORM

I've moved `api_form.xml` that only contains form types definitions for API using `DoctrineORMSerializationType` when using the Doctrine ORM context.

This is to avoid the following issue when using ODM context:

`ParameterNotFoundException: The service "sonata.user.api.form.type.group" has a dependency on a non-existent parameter "sonata.user.admin.group.entity".`

This PR is related to the following comment: https://github.com/sonata-project/SonataUserBundle/commit/9027604673cec6b67b5edfb47cba3ae55fd772e7#commitcomment-5705945

I've also made `serializer.xml` available outside of the API bundles.