Bugfix!  Commit the AIGradingWorkflow model before scheduling a grading task.

First bugfix!  From examining the MySQL logs, I confirmed that Django was committing the transaction after the worker tried to retrieve the workflow.

Fortunately, `AIGradingWorkflow.start_workflow` was already wrapped in a `commit_on_success` decorator, so I moved the code that looks up and sets the classifier into that transaction.  This guarantees that if a classifier set is available, it will be included with the workflow as soon as it is committed to the database.

@stephensanchez