Fix crash while trying to unselect an idea as an author

Caused by missing @Transactional

core/src/main/java/se/su/dsv/scipro/match/IdeaServiceImpl.java

@@ -442,6 +442,7 @@ public class IdeaServiceImpl extends AbstractServiceImpl<Idea, Long> implements
     }
 
     @Override
+    @Transactional
     public void studentUnselect(final Idea idea, final User student) {
         unmatch(idea, student, new NotificationSource());
     }