Will/ai grading updates workflow

[TIM-639](https://edx-wiki.atlassian.net/browse/TIM-639): Students need to load the problem before receiving a score

* Roll the workflow refactor from #423 into the `ai-grading` branch
* Update the AI assessment module to use `on_init()`
* Define a Django signal to allow the AI module to update the workflow.

@stephensanchez