Optimize available_countries

`.parameterize` is a little bit slow. With the existing code, it was run twice
for each comparison. Using `sort_by`, we run it only once per each country.

An [unscientific
benchmark](https://gist.github.com/jhawthorn/e3ff375ff246ddce982f) on a 2.2
sandbox shows this improving `available_countries` from about 165 ms to about
30 ms.

This can be applied as far back as 2-0-stable