Fix homework file size validation & created_by field default value

src/common/serializers.py

src/common/validators.py

src/homework/migrations/0001_initial.py

src/homework/migrations/0009_merge_20180404_0825.py

src/homework/migrations/0010_auto_20180404_0828.py

src/homework/migrations/0011_auto_20180404_0853.py

src/homework/models.py

src/homework/serializers.py

src/homework/views.py