refactor and migration squash

core/custom_annotations.py

core/migrations/0001_initial.py

core/migrations/0001_squashed_0034_auto_20170322_1304.py

core/migrations/0002_auto_20170303_1837.py

core/migrations/0002_auto_20170323_0026.py

core/migrations/0003_auto_20170303_2053.py

core/migrations/0004_submission_final_feedback.py

core/migrations/0005_auto_20170304_1200.py

core/migrations/0006_auto_20170304_1523.py

core/migrations/0007_feedback_slug.py

core/migrations/0008_auto_20170304_1612.py

core/migrations/0009_auto_20170304_1614.py

core/migrations/0010_auto_20170304_1741.py

core/migrations/0011_auto_20170306_1327.py

core/migrations/0012_auto_20170306_1450.py

core/migrations/0013_auto_20170306_1452.py

core/migrations/0014_auto_20170306_1504.py

core/migrations/0015_auto_20170306_1525.py

core/migrations/0016_auto_20170306_1535.py

core/migrations/0017_auto_20170306_1550.py