EDUCATOR-171

EDUCATOR-1400

Add a new waffle switch that allows us to assume zero grades for
learners who have no entry previously recorded, and another to
disable persisting grades for unengaged learners.

TNL-6691

Remove deprecated SingleSectionGrader.  TNL-5987
Remove display_name and module_id from Scores objects
Update CourseGradeFactory.__init__ to not be user-specific
Update some callers to use CourseGrade class instead of "summary" dict
Remove no longer needed course_grades.py module.
Renamed django signal from GRADES_UPDATED to COURSE_GRADE_CHANGED

For TNL-5993

Makes use of the new SUBSECTION_SCORE_CHANGED signal to trigger a task that
updates persisted course grade values. We've also renamed SCORE_CHANGED to
PROBLEM_SCORE_CHANGED to head off any issues with unclear signal names.

TNL-5740

For better user-facing performance, the SCORE_CHANGED signal is now handled by
enqueueing an async task to update the relevant stored grade, rather than
making the request wait until that operation finishes.

TNL-5738

* First take at forcing a subsection's grade to update when a signal is
  sent that a problem's score has changed
* Refactor signal handler connection.
* Expand bokchoy tests to cover progress page
* Add some grading unit tests

TNL-5394
TNL-5364

https://openedx.atlassian.net/browse/TNL-5008

This change allows graded assignments to be added to a campus LMS
regardless of the granularity at which the problem sits. Previously
a grade could only be returned if the usage ID for the problem itself
was specified in the LTI launch.

The code assumes that courses taking advantage of this functionality
are arranged in a hiearchy (with sections being parents to verticals,
and verticals being parents to problems). When a grading event occurs
it traverses the parent hiearchy to identify any previous graded LTI
launches for which the new scoring event should generate a grade
update. It then calculates and sends scores to each of those outcome
services.

Since grade calculation is an expensive operation, the code optimizes
the case where a problem has been added only once as a leaf unit. In
that case it is able to behave as before, just taking the grade from
the signal without having to calculate grades for the whole course.

This change allows the LTI provider to pass grades back to the campus LMS platform using the
LTI outcome service. For full details of the outcome service, see:
    http://www.imsglobal.org/LTI/v1p1/ltiIMGv1p1.html

In brief, the LTI 1.1 spec defines an outcome service that can be offered by an LTI consumer.
The consumer determines whether a score should be returned (in Canvas, this means that the LTI
tool is used in an assignment, and the launch was performed by a student). If so, it sends
two additional parameters along with the LTI launch:

lis_outcome_service_url: the endpoint for the outcome service on the consumer;
lis_result_sourcedid: a unique identifier for the row in the gradebook (i.e. the tool/student/assignment combination).

The LTI Provider launch view detects the presence of these optional fields, and creates database
records for the specific Outcome Service and for the graded LTI launch. Later, when a score on
edX changes (identified using the signal mechanism from previous LTI Provider pull requests),
a Celery task is launched to pass the score back to the LTI consumer.