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.
"Existing problems are unaffected" -> this is not my experience. I set this setting to "true" and immediately all my pre-existing problems got a reset button.