XD-1802: Add @XmlRootElement to all resources

Add @XmlRootElement to all REST resources, so that they don't throw marshalling errors when trying to marshal to xml (e.g. by default in a browser).

Note that this does not make our API xml friendly (by default, all fields are exposed to json by json, but jaxb does the inverse: fields/getters need to be explicitly declared as to-be-marshalled). Created https://jira.spring.io/browse/XD-1800 for that, if we ever want to invest time in it