Fix deleting labs that have a presentation

Closes #753 (closed) .

how to test

create a lab -> go to present it -> actually present the slideshow (so a presentation gets created) -> go back to sessions page -> delete newly created lab

this should not lead to:

as it currently does on dev.

Also, I don't think this needs migrations?

src/main/java/nl/tudelft/queue/model/labs/Lab.java

@@ -89,7 +89,7 @@ public abstract class Lab extends QueueSession<LabRequest> {
 	@Builder.Default
 	private Boolean enableExperimental = false;
 
-	@OneToOne(mappedBy = "lab")
+	@OneToOne(mappedBy = "lab", cascade = CascadeType.ALL, orphanRemoval = true)
 	@ToString.Exclude
 	@EqualsAndHashCode.Exclude
 	private Presentation presentation;