Support and tests for adding a reset button to units

@jbau @jrbl  to review please

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.

Implementation is in common/lib/xmodule/xmodule/capa_base.py  
Refactored out common constants in common/lib/xmodule/xmodule/capa_base_constants.py
Unit tests in common/lib/xmodule/xmodule/tests/test_capa_module.py
UI acceptance tests in common/test/acceptance/tests/test_problem_reset.py 
This comment is probably better as an inline comment on your diff here on github, right? 

This comment appears to be intended for reviewers, not for people reading the code. Because in the future, when people are reading the code, self.done won't be present, and so this comment will have a nonexistent referant, making it not make sense.