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.

One way to look at this (for anyone bored or into comp sci) is that we were running `parameterize` `O(N log N)` times (being the number of comparisons sort will do). With sort_by we will run `parameterize` `O(N)` times and only run the (much faster) string comparison `O(N log N)` times.

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