Allowed option for labs to accept students without a mentor group

...java/nl/tudelft/ewi/queue/model/FirstYearMentorGroup.java

...ain/java/nl/tudelft/ewi/queue/model/FirstYearStudent.java

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

src/main/java/nl/tudelft/ewi/queue/service/LabService.java

src/main/resources/templates/lab/create.html

src/main/resources/templates/lab/edit.html

src/main/resources/templates/lab/enqueue.html

src/main/resources/templates/lab/view.html