Allow multiple feedback options to be selected

* Update `AssessmentFeedback` model to allow selection of zero or more `AssessmentFeedbackOption`s
* Change grade complete template to use checkboxes and wording from [TIM-367](https://edx-wiki.atlassian.net/browse/TIM-367)
* Update JavaScript and XBlock handler to receive feedback on submissions.

UI updates for success/error of submission is still not implemented.  Tracked in JIRA as [TIM-337](https://edx-wiki.atlassian.net/browse/TIM-337)

@jrbl This includes your changes from #167, squashed to one commit and rebased onto master.
@stephensanchez Please review.