apps.core.tests.assignment: Removed unused testhelper.
devilry_gradingsystem: Started working on the new FeedbackDraft model.

See #465.
apps.core.models.assignment|pointrange_to_grade: Added fields for #511.

Added ``max_points``, ``passing_grade_min_points`` and
``points_to_grade_mapper``. Implemented the ``PointRangeToGrade``
model with tests for the constrains.