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
Small changes to the grades djangoapp
-sets up mock_get_score context handler
-fixes an issue with not invalidating lazy scores property
-code quality fixes
-query count optimization in PersistentSubscetionGrade.create
-adds atomic blocks to avoid IntegrityErrors corrupting an entire request
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.
