Allow timeslots to be made empty

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src/main/java/nl/tudelft/queue/model/TimeSlot.java

@@ -80,7 +80,7 @@ public class TimeSlot {
 	 * The capacity of the time slot. Sometimes many people can enrol for one time-slot because many TAs are
 	 * available, sometimes very little are available.
 	 */
-	@Min(1)
+	@Min(0)
 	@NotNull
 	private int capacity;