Added jpa-and-mongodb demo app

This app demonstrates how to mix Spring Data JPA and Spring Data MongoDB together (with Spring Data REST on top) to demonstrate how to split them up properly.

This was motivated by https://github.com/spring-guides/gs-accessing-data-mongodb/commit/98f252468932d205e6f3524f16b1269285a680c0#commitcomment-7105530 where a user tried to mix two guides together.