Language and country tags should recognize uppercase letters

In the setup in index.md:

```
country: "nz"      # lowercase two-letter ISO country code such as "fr" (see https://en.wikipedia.org/wiki/ISO_3166-1#Current_codes) for the institution that hosts the workshop
language: "en"     # lowercase two-letter ISO language code such as "fr" (see https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes) for the workshop
```

Should be as valid as `NZ` and `EN`. 