Resolve "Provide import functionality via frontend for reviewers"

Closes #189 (closed)

core/serializers/__init__.py

 from .common_serializers import *  # noqa
 from .submission_type import (SubmissionTypeListSerializer, SubmissionTypeSerializer,  # noqa
                               SolutionCommentSerializer)  # noqa
-from .feedback import (FeedbackSerializer, FeedbackCommentSerializer,  # noqa
+from .feedback import (FeedbackSerializer,  # noqa
+                       FeedbackCommentSerializer,  # noqa
                        VisibleCommentFeedbackSerializer)  # noqa
 from .subscription import *  # noqa
 from .student import *  # noqa