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