Added FeedbackLabel model, serializer and View

core/migrations/0014_feedbacklabel.py

core/models/__init__.py

core/models/exam_type.py

core/models/feedback.py

core/models/label.py

core/models/submission.py

core/permissions.py

core/serializers/__init__.py

core/serializers/feedback.py

core/serializers/label.py

core/tests/test_feedback.py

core/tests/test_labels.py

core/urls.py

core/views/__init__.py

core/views/label.py