Update patient model parse handling of race and ethnicity

### Story ###
- https://www.pivotaltracker.com/story/show/79806350

### Notes ###
- ```race``` and ```ethnicity``` are extracted from the backend hash/object representation, which happens twice when loading the ```deepClone()```'d model in the patient builder
- Note: the desired race and ethnicity were always being saved correctly, but not rendered when editing the patient within the patient builder
This code might be a bit confusing outside the context of the pull request; a comment would help, but maybe a different change would be useful:

    attrs.ethnicity_code = attrs.ethnicity?.code
    attrs.race_code = attrs.race?.code

This would require changes in other places too (at least the controller and the form, probably calculation code too), so might not be worth it...