Feature/completion exercise

This PR adds the ability to define completion exercises. These are exercises that require the student to complete the chord that is displayed on the screen, with the help of a prompt. The exercises are designed such that an instructor can specify the "visible" and "hidden" parts of the chord to be completed.

@jazahn Can you review this?

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[HLAB-74](https://jira.huit.harvard.edu/browse/HLAB-74)