Add TrackedHistoryEntty mapped superclass

We need a class that will allow storing a version of itself for conflict resolution.

@MappedSuperclass
public class  TrackedHistoryEntty {

  @JsonIgnore
  @ElementCollection
  public List<String> historyJson;

  @PreUpdate
  @PrePersist
  public void addHistory() {
      // serialize this in JSON and add it in the historyJson list
  }

  public <T extend TrackedHistoryEntty> List<T> getHistory() {
    // deserialize the historyJson and return it as a list. 
  }
}

All entities that are updated by the professors should extend this class:

• CoursePreference
• JoinedSubject
• ProfessorEngagement
• SubjectDetails
• ExamDefinition