enforce serverside check for timeslots

src/main/java/nl/tudelft/queue/service/RequestService.java

@@ -139,6 +139,11 @@ public class RequestService {
 			 *  and let it generate a link for the respective online mode.
 			 * This is for the far future, when more online modes need to be supported.
 			 */
+
+			if (labRequest.getTimeSlot() != null && !labRequest.getTimeSlot().canTakeSlot()) {
+				throw new AccessDeniedException("Time slot is not available");
+			}
+
 			if (((LabRequest) request).getOnlineMode() == OnlineMode.JITSI) {
 				labRequest.setJitsiRoom(js.createJitsiRoomName(labRequest));
 			}