Add assignment_completed? to Submission

Not the greatest place for the method, but it's fairly clean. Adding to
give the notification API ability to determine if the assignment is ongoing and worth
caring about.
Add assignmentCompleted to Notification API
Gives thumbs-up for completed nitpicks notifications

When a user gets a nitpick for an assignment that's completed then a
thumbs up icon is displayed.
Merge pull request #445 from kytrinyx/notifation-enchancements

Notification Wish List