Add checkbox option for participant country to "All Responses" download

# TL;DR

In the "All Responses" study view (https://lookit.mit.edu/exp/studies/###/responses/all/), add a checkbox to the list of characteristics that includes the family's country of residence.

# Narrative
As a researcher, I want to be able to see the location of each participant with their responses so I can pay them in the correct currency

# Acceptance Criteria

- [ ] Ensure country field from User data is appropriate for use as place-of-residence
- [ ] Add country to set of fields available for addition to Response object (if not already)
- [ ] Checkbox added to front end
- [ ] Tests for new feature
- [ ] QA on new feature
- [ ] Sign off from Kim

# Implementation Notes

Start at the All Responses study view and trace how User data is made available on the Response object and how the existing characteristic checkboxes drive downloads. Verify the country field represents place of residence, add the checkbox and coverage in the tests, then complete QA and obtain Kim's sign-off.

Written by the indexing model from the issue text.