Filter countries with <15k population

[scholtzan]

Countries with less than 15k in population should be filtered out, similar to like it's done for cities. A dataset with population per country is available here: http://download.geonames.org/export/dump/countryInfo.txt

[jklukas]

cc @whd

[whd]

I filed https://bugzilla.mozilla.org/show_bug.cgi?id=1619448 which is somewhat related. I would like us to formalize and document a bit more our usage of geonames.