Added scoreboard that ranks countries based on number of events

Created a new /scoreboard page that has a list of countries and the number of events in each country (#251). The list also takes into account the population of each country, so that smaller countries with more events per citizen stand a chance of climbing to the top.

To get population data, I added django-countries-plus. The page could use some design love and to be linked from elsewhere, although we can secretly deploy it first and see how people react to it :grin: 
Thanks, added an extra check!