Support and tests for adding a reset button to units

Decouples reset from randomization while still preserving backward compatibility. Users will be able to effectively clear their input for answere questions that they have not submitted and instructors are able to set course-wide and problem-specific settings.
Merge branch 'caijim/resetbutton' of github.com:Stanford-Online/edx-platform into caijim/resetbutton
Incorporated @jrbl's feedback and removed acceptance tests.

Todo: Add unit test that tests for existence of button# Please enter the commit message for your changes. Lines starting