Resolve "Resolve issues with deleting an edition"

What does this MR do (summary)?

Make sure role's get removed for a deleted session in order to prevent other applications from trying to access them whilst accessing the roles entries.

Screenshots

What changes have you made (detailed)?

Does this MR meet the acceptance criteria?

• The main application was run to make sure the server still runs without errors.
• Tests were written to verify the behaviour of this code.
• Javadoc and other comments were added to make the code understandable.
• The code adheres to the style guidelines.

Closes #145 (closed)

src/main/java/nl/tudelft/labracore/model/Edition.java

@@ -103,7 +103,7 @@ public class Edition {
 	@Builder.Default
 	@ToString.Exclude
 	@EqualsAndHashCode.Exclude
-	@OneToMany(mappedBy = "edition")
+	@OneToMany(mappedBy = "edition", orphanRemoval = true)
 	private Set<Role> roles = new HashSet<>();
 
 	@Builder.Default