Allow inputting/searching countries and locales by their codes rather than full names

[aminomancer]

Sometimes we're given a long list of country or language codes, or they might be pulled from a list of supported regions for a feature in source code (example). And since Experimenter requires inputting the full names, you have to spend some time figuring out the corresponding name for each code. Not the end of the world, but still a papercut.

I believe this field is a frontend for adding targeting in the form of language/region codes. If we start from a list of codes and the final output is the same list of codes, then there's no need for the middleman. But I don't propose eliminating the full names, since those are probably used often. I think the field is based on react-select, and it has some kind of fuzzy search algo, so the entries could be like this:

{
  value: "United Kingdom GB",
  label: "United Kingdom"
}

Or the label could include the code too, like United Kingdom (GB). But that is just to show that you don't need to visually display the code to make the entries searchable by code.

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