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 
What happens when self.rerandomize is "per_student"?

The comment on 1445 also appears to be one intended for reviewers rather than one meant to be in the commit? 

I'd either drop it, or edit it so it makes sense to the people of the future. Perhaps something like
```
# Update random number seed on rerandomize and submit
```
...but that's actually as sensible from the code as anything in capa is, so it's not clear such a comment helps scrutability much.