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 
I think this can be tightened up by relying on the conventions employed by similar settings above:
````
Enter true or false. If true, problems default to displaying a 'Reset' button. This value may be overriden in each problems' settings. Existing problems are unaffected.
```

It's not so much that I think a terse style is better; it's more that I think every word should be made to justify itself.