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.
Phase 1 of adding asset metadata saving to old Mongo
Merge pull request #5595 from Stanford-Online/caijim/resetbutton

Support and tests for adding a reset button to units
Merge pull request #4854 from edx/jeskew/assetstore_modulestore_work

Phase 1 of adding asset metadata saving to old Mongo