Update DB to use string UUID as sandboxId, added documentation to TrainingRun

src/main/java/cz/muni/ics/kypo/training/adaptive/domain/training/TrainingRun.java

@@ -128,14 +128,14 @@ public class TrainingRun implements Serializable {
     )
     @JoinColumn(name = "training_instance_id")
     private TrainingInstance trainingInstance;
-    @Column(name = "sandbox_instance_ref_id")
+    @Column(name = "sandbox_instance_ref_id", length = 36)
     private String sandboxInstanceRefId;
     @ManyToOne(fetch = FetchType.LAZY)
     @JoinColumn(name = "user_id", nullable = false)
     private User participantRef;
     @Column(name = "phase_answered")
     private boolean phaseAnswered;
-    @Column(name = "previous_sandbox_instance_ref_id")
+    @Column(name = "previous_sandbox_instance_ref_id", length = 36)
     private String previousSandboxInstanceRefId;
     @ElementCollection(targetClass = SolutionInfo.class, fetch = FetchType.LAZY)
     @CollectionTable(name = "solution_info", joinColumns = @JoinColumn(name = "training_run_id"))

src/main/resources/db/migration/V10__db_sandbox_id_to_string.sql

+alter table training_run alter column sandbox_instance_ref_id type varchar (36);
+alter table training_run alter column previous_sandbox_instance_ref_id type varchar (36);