Allow users to change role through imports if they are already in the course

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

@@ -18,6 +18,7 @@ package nl.tudelft.ewi.queue.service;
 import com.google.common.collect.Lists;
 import com.querydsl.core.types.dsl.BooleanExpression;
 import nl.tudelft.ewi.queue.helper.CsvUserHelper;
+import nl.tudelft.ewi.queue.repository.CourseRepository;
 import nl.tudelft.ewi.queue.model.Assistant;
 import nl.tudelft.ewi.queue.model.Course;
 import nl.tudelft.ewi.queue.model.DefaultRole;
@@ -51,6 +52,7 @@ import java.time.LocalDateTime;
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.List;
+import java.util.Optional;
 import com.google.common.collect.Lists;
 import com.querydsl.core.types.dsl.BooleanExpression;
 import java.io.IOException;