Build a better way to select date/time settings

`/wp-admin/options-general.php` includes manual settings for date format, time format, and day that the week starts on.

These can be replaced with a single drop down for a country since CLDR provides all that data.

Start at /wp-admin/options-general.php and trace how the date format, time format, and week-start settings are currently presented and stored. Review the CLDR data available to the project, then define the country-selection flow and verify that it supplies all three settings consistently.

Written by the indexing model from the issue text.