DATAREST-238: Change JSON deserializer so it ignores _links

_links are read-only and should basically be ignored during any inputs. This required removing the deserializer from PersistentEntityJackson2Module. A side effect is that any entity would crash based on the missing _links attribute. Instead of requiring that every entity apply the necessary annotation, the mapper is configured to not fail on non-existent attributes. This allowed all the tests to pass while properly handling PUT operations.