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 
If you give your tests a triple-quoted one-line summary docstring, it will be output when the test fails, which can be helpful when debugging:
```
"""Test Re-randomization on submit with no reset button."""
```
This is also helpful to readers who may have TroubleReadingTheCamelCasedClassNames.