Allow country endpoint to return more than one filter country

[ahalterman]

Mordecai limits the search for place names to a single country per document, to reduce errors and to avoid the US bias in other systems. However, articles often reference locations in more than one country. Allow Mordecai to pick more than one country in certain cases. This will rely on having lots of high-quality labeled text data.

[ahalterman]

New architecture will use the document country as a single input to a model, rather than a hard rule.