Django 2.0 compatibility

cosinnus_todo/dashboard.py

cosinnus_todo/notify.py

tests/view_tests/base.py

tests/view_tests/test_add.py

tests/view_tests/test_assign.py

tests/view_tests/test_assign_me.py

tests/view_tests/test_complete.py

tests/view_tests/test_delete.py

tests/view_tests/test_detail.py

tests/view_tests/test_edit.py

tests/view_tests/test_index.py

tests/view_tests/test_list.py

tests/view_tests/test_unassign.py