Use Winkel-Tripel projection for countries map#146

The Mercator projection currently used in the countries map does not accurately represent the geographic size of countries. Since land mass has some correleation to population of a country, this projection overstates the potential audience size of each country. See https://thetruesize.com/ for a fun interactive example of this distortion.

The Winkel-Tripel is the projection preferred by the National Geographic Society. From a mathematical perspective, it may be the most correctly balanced with respect to the various types of distortion introduced by map projections: area, isotropy (local scaling), flexion (bending), and skewness (lopsidedness).

The Goode homolosine projection would also be a better choice than Mercator, but it splits Greenland in two.

Thank you for your contribution. We’re moving this thread to Github: https://github.com/plausible/analytics/issues/174