Remove entity manager references from services and convert their tests to unit tests

We are currently doing integration testing on services. Convert these to unit tests.

In order to achieve this, we should move the persistence context reference to repositories, using the custom repository pattern: http://docs.spring.io/spring-data/jpa/docs/current/reference/html/#repositories.single-repository-behaviour

We should also make sure, that usage of the entity manager is justified and cannot be replaced by generated queries.