Support and tests for adding a reset button to units

Users may want to start anew when answering a question. This commit decouples reset from randomization while still preserving backward compatibility. Users can clear their input.
Instructors can set course-wide and problem-specific settings for reset button display.
Sure - just be sure you reply in the appropriate place. If you reply in an unrelated section of code, when you address my original comment your follow up comments disappear.