Users currently store a country, so it would be helpful to list all users in a country in a server. Example of the command could be
ls <country name or iso code>

Which then returns (probably in embed so the users don't get pinged) the users of that country alphabetically ordered.

• Country class
• [Toggle count

online-judge-tools/oj#637

By the way, is assertion in download_sample_cases is right?
If assert len(list(case.children)) == 2 is not satified, the page contents are not supported by oj.
Therefore, raise SampleParseError is prefer to assertion?

Anyway, this assertion issue handle by other PR. This PR is mergable.

アサーションはバグ検出のために仕込むものですが、`assert len(list(case.childr