Switch to using ISO3 and UN M.49 codes

[alanorth]

It would be very nice if we could switch to using standardized codes for countries and regions, ie:

• ISO 3166-1-alpha-2 country codes
• ISO 3166-1 alpha 3 country codes
• UN M.49 area codes

I think we could do this by simply using different displayed/stored values in the item submission form, for example in input-forms.xml:

<pair>
    <displayed-value>AFGHANISTAN</displayed-value>
    <stored-value>AF</stored-value>
</pair>

[alanorth]

As of mid-2020 we are now using mostly ISO 3166-1 country names in our submission forms and tagging ISO 3166-1 Alpha2 country codes via a nightly curation task. More recently we began using UN M.49 for region names.