Django 2.0 will make this field required for `ForeignKey` and `OneToOneFields`.
In previous versions the option defaulted to `models.CASCADE` when not
specified. This change should make the deprecation warnings in the current
Django version go away.
The migrations where also modified, but the changes should not cause a change in
the database schema since `models.CASCADE` was already the old default.
Some old mongo courses do not have this field, and the team has opted
to allow null values rather than inserting a default. This change
affects both course and subsection grades.
TNL-6408
The column is populated with the current timestamp when a passing grade
is persisted for the first time. "Passing grade" is defined as having a
non-blank value for letter_grade.
TNL-5888
With this change, it is now possible to enable and disable the feature for a
particular course as many times as desired. Fix confirmed locally.
TNL-5698
Hooks the pre-existing grades code into our new PersistentSubsectionGrade data
model. Includes test updates, and some minor changes to the data model that
were discovered in testing.
