refactor: no mutating countries/timezones choices

[jennifer-richards]

This should preserve exactly the previous behavior. If tests fail complaining about needing a migration, then something went wrong (it did not want a migration locally, so I don't expect it to here)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.70%. Comparing base (18cea10) to head (baa6507).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9037      +/-   ##
==========================================
- Coverage   88.71%   88.70%   -0.02%     
==========================================
  Files         320      320              
  Lines       41793    41787       -6     
==========================================
- Hits        37078    37066      -12     
- Misses       4715     4721       +6     

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