Resolve "Enqueue page allowed to be visited while already enqueued"

What does this mr do?

Visiting the enqueue page, while already enqueued, does not redirect to the lab.

Actions taken to fix bug

Write a check in the LabController to check if the user is already enqueued. If yes, the user is sent back to the /lab/{id}

Does this MR meet the acceptance criteria?

• I have added a changelog entry to reflect the significant changes I made and the bug I fixed.
• A test was created to test the bug.
• I have updated the documentation accordingly.
• I adhere to the style guide.

src/main/java/nl/tudelft/ewi/queue/controller/LabController.java

@@ -137,9 +137,11 @@ public class LabController {
 
 	@GetMapping("/lab/{id}/enqueue")
 	@PreAuthorize("@permissionService.canEnqueueSelfForLab(principal, #id)")
-	public String enqueue(@AuthenticatedUser User user,
-			@PathVariable("id") Long id, Model model) {
+	public String enqueue(@AuthenticatedUser User user, @PathVariable("id") Long id, Model model) {
 		Lab lab = labService.getLab(id);
+		if (lab.isEnqueued(user)) {
+			return "redirect:/lab/" + id;
+		}
 
 		if (lab.isSignOffIntervals()) {
 			model.addAttribute("timeslots", labService.getIntervalsForLab(lab));
@@ -157,6 +159,9 @@ public class LabController {
 	public String enqueue(@AuthenticatedUser User user, @PathVariable("id") Long id,
 			@Valid RequestCreateModel requestCreateModel, BindingResult bindingResult, Model model) {
 		Lab lab = labService.getLab(id);
+		if (lab.isEnqueued(user)) {
+			return "redirect:/lab/" + id;
+		}
 
 		requestCreateModel.setLab(lab);
 		if (lab.getCourse().getHasGroups()) {