Fix some issues with questionnaire model

...n/java/com/example/demo/domain/QuestionPhaseRelation.java

...main/java/com/example/demo/domain/QuestionnairePhase.java

src/main/java/com/example/demo/dto/AbstractQuestionDto.java

...n/java/com/example/demo/dto/QuestionPhaseRelationDto.java

...mple/demo/repository/QuestionPhaseRelationRepository.java

...a/com/example/demo/service/QuestionnairePhaseService.java