Unanswered on input

Description
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When a student inputs an answer and checks the problem, it is shown as correct or incorrect (by the appearance of a check or a red X). Prior to this PR, the "correctness" stays, even when the student later changes their answer. This produces a confusing visual when they have a (possibly incorrect) answer with a green check mark, or vice versa, etc&.

This PR seeks to add event handling to each of the input types so when input occurs or a change happens, that the "correctness" is returned to "unanswered".

Reviewers
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* ???

Merger
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(I, Peter, am leaving edX as the summer ends. I suppose someone else will be the official sponsor of this request. This person is...)